Introduction to Microcomputer
Microcomputer Structure and Operation
A microcomputer is a small, low cost digital computer, which usually consist of a microprocessor, a storage unit, an input channel and an output channel, all of which may be one chip inserted into one of several PC boards
Von Neumann model of computer architecture
The major parts of this model are the Central Processing Unit (CPU), memory,and the lnput
and output circuitry or I/O. Connecting these parts are three sets of parallel lines called buses. The three buses are the Address Bus, the Data Bus, and the Control Bus.Memory:
The memory section usually consists of a mixture of RAM (Random Access Memory) and ROM (Read Only Memory). It may also have magnetic floppy disks, magnetic hard disks, or optical disks (CDs, DVDs).Memory has two purposes. The first purpose is to store the binary codes for the sequences of instructions you want the computer to carry out. When we write a computer program, what we are really doing is writing a sequential list of instructions for the computer. The second purpose of the memory is to store the binary-coded data with which the computer is going to be working
Input/Output:
The Input/output or I/O sections allow the computer to take in data from the outside world or send data to the outside world. Peripherals such as keyboards, video display terminals, printers, and modems are connected to the I/O section. These allow the user and the computer to communicate with each other. The actual physical devices used to interface the computer buses to external systems are often called ports
Ports in a computer function just as shipping ports do for a count An input port allows data from a keyboard, an A/D converter, or some other source to be read into the computer under control of the CPU. An output port is used to send data from the computer to some peripheral, such as a video display terminal, a printer, or a D/A converter
Data Bus:
The data bus consists of 8, 16, or 32 parallel signal lines. As indicated by the double-ended arrows on the data bus line. The data bus lines are bidirectional This means that the CPU can read data in from memory or from a port on these lines, or it can send data out to memory or to a port on these lines
Many devices in a system will have their outputs connected to the data bus but only one device at a time will have its outputs enabled. Any device connected on the data bus must have three-state outputs so that its outputs can be disabled when it is not being used to put data on the bus.
Address Bus:
The address bus consists of 16, 20, 24 or 32 parallel signal lines. On these lines the CPU sends out the address of the memory location that is to be written to or read from. The number of memory locations that the CPU can address is determined by the number of address lines.
If the CPU has N address lines. then it can directly address 2N memory locations. For example, a CPU with 16 address lines can address 216 or 65.536 memory locations, a CPU with 20 address lines can address 220 or 1.048.576 locations, and a CPU with 24 address lines can address 224 or 16,777216 locations. When the CPU reads data from or writes data to a port, it sends the port address out on the address bus.
Control Bus:
The control bus consists of 4 to 10 parallel signal lines. The CPU sends out signals on the control bus to enable the outputs of addressed memory devices or port devices. Typical control bus signals are Memory Read,Memory Write, I/O Read, and I/O Write. To read a byte of data from a memory location.
For example, the CPU sends out the memory address of the desired byte on the address bus and then sends out a Memory Read signal on the control bus. The Memory Read signal enables the addressed memory device to output a data word onto the data bus. The data word from memory travels along the data bus to the CPU.
Central Processing Unit (CPU):
The Central Processing Unit or CPU controls the operation of the computer. In a computer the CPU is a microprocessor. The CPU fetches binary-coded instructions from memory, decodes the instructions into a series of simple actions, and carries out these actions in a sequence of steps.
The CPU also contains an address counter or instruction pointer register which holds the address of the next instruction or data item to be fetched from memory; general-purpose registers, which are used for temporary storage of binary data: and circuitry, which generates the control bus signals
Hardware, Software, and Firmware:
When working around computers, we hear the term hardware, software, and firmware almost constantly. Hardware is the name given to the physical devices and circuitry of the computer. Software refers to the programs written for the computer. Firmware is the term given to programs stored in ROMs or in other devices which permanently keep their stored information.
Introduction to Microprocessor
Microprocessor is a clock driven semiconductor device. It can be manufactured by using either a large scale integration (LSI) or very large scale integration (VLSI) technique. The microprocessor is similar to CPU, but it includes all the logic circuitry, including the control unit. The micro processor contains three segments namely arithmetic logic unit (ALU),register array and control unit.
The microprocessor is capable of performing various computing functions and making decisions to change the sequence of program execution.
Working Principle of a Microprocessor:
It is the central processing unit which coordinates all the functions of computer. It generates timing Signals, sends and receives data to and from every peripheral used inside or outside the computer.
The commands required to do this are fed into the device in the form of current variations which are converted into meaningful instructions by the use of a Boolean Logic System. It divides its functions in two categories, logical functions and processing functions.
The arithmetic and logical unit and the control unit handle these functions respectively. The information is communicated through a bunch of wires called buses. The address bus carries the 'address' of the location with which communication is desired while the data bus carries the data that is being exchanged.
ALU:
In this area various computing functions are performed on data. ALU unit performs such Arithmetic operations as addition, subtraction and such logic operations as AND,OR,Exclusive OR.
Register array:
Microprocessor consists of various registers identified by letters such as B,C,D,E,H and L. It is used to store the data temporarily during execution of program.
Control unit:
It provides necessary timings and control signals to all the operations in the micro computer. It controls the flow of data between the microprocessor and memory and pheripherals.
Applications of Microprocessor:
Microprocessor is a multi-use device which finds applications in almost all the fields. Here is some sample applications given in variety of fields.
Electronics: Digital clocks & Watches, Mobile phones, Measuring Meters.
Mechanical: Automobiles, Lathes, All remote machines.
Electrical: Motors Lighting controls, Power stations.
Medical: Patient monitoring, Most of the Medical equipments, Data loggers.
Computer: All computer accessories, Laptops & Modems, Scanners and Printers.
Domestic: Microwave Ovens, Television/CD/DVD players,Washing Machines.
Architecture of 8085
8O85 microprocessor consists of various functional blocks as listed below
(i) Register
· General Purpose Register
· Temporary Register
· Special Purpose Register
--Accumulator
--Flag Register
--Instruction Register
--Memory pointer register
(ii) Arithmetic Logic Unit(ALU)
(iii) Instruction Decoder and Machine Cycle Encoder
(iv) Address Buffer
(v) Data/Address Buffer
(vi) Incrementer/Decrementer Address Latch
(Vii) Interrupt Control
(Viii) Serial 1/O Control
(ix)Timing and Control Unit
Registers of 8085:
Register is a group of binary cells. In 8085 the registers are divided into three categories
· General Purpose register
· Temporary Register
· Special Purpose Register
General Purpose Register
· The 8085 have six general purpose register to store s-bit data during program execution.
· These registers are identified as B,C,D,E, H and L.
· They can be combined as register pairs BC,DE and HL to perform some 16-bit operations.
Temporary Register
It is an 8-bit register. It is used to hold the data during the arithmetic and logical operations. The various temporary registers in 8085 are W and Z register. It is used for temporary storage of data during the execution of an instruction.
Special Purpose Register:
Accumulator (A)
· The accumulator is an 8-bit register that is part of the arithmetic logic unit (ALU).
· This register is used to store 8-bit data and to perform arithmetic and logical operations.
· The result of an operation is stored in the accumulator.
Flag Register
The ALU includes five flip-flops that are set or reset according to the result of an operation. The flags are affected by the arithmetic and logical operations in the ALU .The microprocessor uses the flags for testing the data conditions. The various flag register in 8085 are as follows:
· Sign flag (S)
· Zero flag(z)
· Auxiliary Carry flag (AC)
· Parity flag (P)
· Carry flag (CY)
The most commonly used flags are Sign, Zero and Carry.
The bit position for the flags in flag register is.
|
D7 |
D6 |
D5 |
D4 |
D3 |
D2 |
D1 |
D0 |
|
S |
Z |
|
AC |
|
P |
|
CY |
S-Sign Flag
After execution of any arithmetic and logical operation, if D7 of the result is 1, the sign flag is set. Otherwise it is reset.
D7 is reserved for indicating the sign ;the remaining is the magnitude of
Number.
if D7 is 1,the number will be viewed as negative number. If D7 is 0, the
number will be viewed as positive number.
Z-Zero Flag
If the result of arithmetic and logical operation is zero, then zero flag is set otherwise it is reset.
AC - Auxiliary Carry Flag
In an arithmetic operation, when a carry is generated by digit D3, and passed onto digit D4, the AC flag is set, otherwise it is reset.
P- Parity Flag
If the result of arithmetic and logical operation contains even number of 1's then this flag will be set and if it is odd number of 1's it will be reset.
C-Carry Flag
If any arithmetic and logical operation result any carry then carry flag is set otherwise it is reset.
Instruction Register
When an instruction is fetched from the memory, it is loaded in the instruction register.
Memory Pointer Register:
Program Counter (PC)
· This 16-bit register sequencing the execution of instructions.
· It is a memory pointer. Memory locations have 16-bit addresses.
· The function of the program counter is to point to the memory address of the next instruction to be executed.
· When an opcode is being fetched, the program counter is incremented by one to point to the next memory location.
Stack Pointer (SP)
The stack pointer is also a 16-bit register used as a memory pointer.
· It points to a memory location in R/W memory, called the stack.
· The beginning of the stack is defined by loading a 16-bit address in the stack pointer (register).
Arithmetic and Logic Unit (ALU)
· It is used to perform the arithmetic operations like addition,subtraction, multiplication, division, increment and decrement and logical operations like AND, OR and EX-OR.
· It receives the data from accumulator and registers.
· According to the result it set or reset the flags.
Instruction Decoder:
It gets the instruction from the instruction register and decodes the instruction It identifies the instruction to be performed.
Machine Cycle Encoder:
The 8085 microprocessor executes different types of machine cycles. It
gives the information about which machine cycle is currently executing in the encoded form on the S0,S1and IO/M lines.
Address Buffer:
· The address bus is a group of 16 lines generally identified as A0 to A15.
· The address bus is unidirectional: bits flow in one direction from the MPU to peripheral devices.
Data Buffer:
· The data bus is a group of eight lines used for data flow.
· These lines are bi-directional - data flow in both directions between the MPU and memory and peripheral devices.
· The eight data lines enable the MPU to manipulate 8-bit data ranging from 00 to FF (28 = 256 numbers).
Incrementer/Decrementer Address Latch:
This 16-bit register is used to increment or decrement the contents of program counter and stack pointer.
Interrupt Control:
Interrupts are signals coming from external devices to call the attention
of the microprocessor. The 8085 has five interrupt signals. They are TRAP, RST7.5, RST 6.5, RST 5.5 and INTR.
Serial 1/0 Control:
The 8085 microprocessor provides two serial 1/0 controls.
(i) SID (Serial Input Data) - This line is used to receive data bit by bit(serially).
(ii)SOD(Serial Output Data) - This line is used to send data bit by bit (serially).
Timing and Control Unit:
This unit synchronizes all the microprocessor operations with the system clock.It also generates the control signals necessary for communicating in between the microprocessor and peripherals.
PIN Configurations Of 8085
8085 is a 40 pin IC. DIP package. All the signals can be classified into five groups.
1. Power supply and clock signals
2. Address bus
3. Data bus
4. Control and status signals
5. Interrupts and externally initiated signals
Power supply and Clock frequency signals:
Vcc: +5 volt power supply
Vss: Ground terminal
XI, X2: Crystal or R/C network or LC network connections to set the frequency of internal clock generator. The frequency is internally divided by two. Since the basic operating timing frequency is 3MHz ,a 6 MHz crystal is connected externally.
CLK -Clock Output is used as the system clock for peripheral and devices interfaced with the microprocessor.
Address Bus:
A8- A15
· It carries the most significant 8 bits of the memory address or the 8 bits of the I/O address.
Multiplexed Address / Data Bus:
AD0 - AD7
· These multiplexed set of lines used to carry the lower order 8 bit address as well as data bus.
· During the opcode fetch operation, in the first clock cycle, the lines deliver the lower order address A0-A7.
· In the subsequent I/O memory, read/write clock cycle the lines are used as data bus.
· The CPU may read or write out data through these lines.
Control and Status signals:
ALE -Address Latch Enable
· This signal helps to capture the lower order address presented on the multiplexed address/data bus.
RD (Read)- Read memory or IO device:
· This indicates that the selected memory location or l/O device is to be read and that the data bus is ready for accepting data from the memory or I/O device.
WR (Write) - Write memory or IO device:
· This indicates that the data on the data bus is to be written into the selected memory location or I/O device
IO/M (InputOutput/Memory) - Select memory or an 10 device.
· This status signal indicates that the read/write operation relates to whether the memory or I/O device
· It goes high to indicate an I/O operation.
· It goes low for memory operations.
S0 and S1 - These are status signals, similar to IO/M. These signals can be used to identify various machine cycle operations.
Interrupts and Externally initiated operations
They are the signals initiated by an external device to request the microprocessor to do a particular task or work. There are five hardware interrupts called
TRAP
RST 7.5
RST 6.5
RST 5.5
INTR
INTA ( active low output)
On receipt of an interrupt, the microprocessor acknowledges the interrupt by the active low INTA (Interrupt Acknowledge) signal.
Reset In:
· This signal is used to reset the microprocessor.
· The program counter inside the microprocessor is set to zero.
· The buses are tri-stated.
Reset Out:
· It indicates CPU is being reset.
· Used to reset all the connected devices when the microprocessor is reset.
READY:
· Memory and I/O devices will have slower response compared to microprocessors.
· Before completing the present job such a slow peripheral may not be able to handle further data or control signal from CPU.
· The processor sets the READY signal after completing the present job to access the data.
· The microprocessor enters into WAIT state while the READY pin is disabled.
Single Bit Serial I/O ports:
SID - Serial input data line
SOD -Serial output data line
These signals are used for serial communication.
Instruction classifications of 8085 microprocessor
An instruction is a binary pattern designed inside a microprocessor to perform a specific function. The entire group of instructions, called the instruction set.
These instructions can be classified into the following five functional categories:
1. Data transfer (copy) operations
2. Arithmetic operations
3. Logical operations
4. Branching operations
S. Machine-control operations
Data Transfer (Copy) Operations:
This group of instructions copy data from a location called a source to another location called a destination, without modifying the contents of the source.
After data transfer the contents of the source could not be varied. Data transfer instructions moves or copies data between registers, between memory and registers and specific data byte to a register or a memory location.
Arithmetic Operations:
These instructions perform arithmetic operations such as addition, subtraction, increment, and decrement.
Addition Any 8-bit number, or the contents of a register or the contents of a memory location can be added to the contents of the accumulator and the sum is stored in the accumulator. No two other 8-bit registers can be added directly. For Example, the contents of register B cannot be added directly to the contents of the register C.
Subtraction - Any 8-bit number, or the contents of a register, or the contents of a memory location can be subtracted from the contents of the accumulator and the results stored in the accumulator. The subtraction is performed in 2’s complement, and the results if negative, are expressed in 2’s compliment. No two other registers can be subtracted directly.
Increment/Decrement - The 8-bit contents of a register or a location can be incremented or decrement by 1. Similarly, the 16-bit content of a register pair such as (BC,DE,HL) can be incremented or decremented by 1. These Increment and decrement operations differ from addition and subtraction in an important way: i.e. they can be performed in any one of the register or in memory location.
· AND, OR Exclusive-OR- Any 8-bit number, or the contents of register, or of a memory location can be logically ANDed, ORed, Exclusive-ORed with the contents of the accumulator. The results are stored in the accumulator.
· Rotate- Each bit in the accumulator can be shifted either left or right to the next position.
· Compare- Any 8-bit number, or the contents of a register, or a memory location can be compared for equality, greater than, or less than, with the contents of the accumulator.
· Complement - The contents of the accumulator can be complemented
All 0’s are replaced by 1’s and all 1’s are replaced by 0’s.
- Jump Conditional jumps are an important aspect of the decision making process in the programming .These instructions test for a certain conditions (eg. Zero or Cary flag) and alter the program sequence when the condition is met. In addition, the instruction set includes an instruction called unconditional jump.
- Call, Return, and Restart- These instructions change the sequence of a program either by calling a subroutine or returning from a subroutine. The conditional Call and Return instructions also can test condition flags.
1. Copying data
2. Performing arithmetic operations
3. Performing logical operations
4. Testing for a given condition and altering the program sequence.
Programming techniques such as Looping, Counting and Indexing And Addressing Modes
The programming techniques used to instruct
the microprocessor to repeat tasks is called looping. A loop is set up by
instructing the microprocessor to change
the sequence of execution and perform the task again. Looping is accomplished
by using jump instructions.
Loops can be classified into two groups
1. Continuous loop
2. Conditional loop
Continuous loop
A continuous loop is set up by using the
unconditional jump instruction.This loop repeats a task continuously. A program
with a continuous loop does not stop repeating the tasks until the system is
reset.
Example Continuous monitor system.
Conditional loop
A conditional loop is set up by the
conditional jump instructions. These instructions checks zero, carry ,flags etc
and repeat the specified tasks if the Conditions are satisfied. These loops
usually include counting and indexing.
Counting
Counting is a programming techniques used to
count the number of execution of a loop for this job a counter will be used.
Counting is always associated with looping. A counter is a typical application
of the conditional loop.
For Example: If the microprocessor wants to repeat a task for five times, the microprocessor
needs a counter, and when counting is completed, it needs a flag for
checking the counter state.
Indexing
Indexing means pointing or referencing objects
with sequential numbers. For example: In a library books are arranged according
to numbers, and they are referred to or sorted by numbers. This is called indexing.
Similarly, data bytes are stored in memory
locations, and those data bytes are referred to by their memory locations.
Addressing modes
The program and data are stored in main memory. Each instruction
of the program specifies the operation to be performed on same data which may in
the registers (or) in memory or in the instruction itself. The way the operands
(the data) are chosen during execution is depended on the addressing modes.
Different techniques are adopted to specify
the data. These techniques are called "addressing modes"
In the 8085, there are five addressing modes are used:
1. Direct addressing
2. Register addressing.
3. Register indirect addressing.
4. Immediate addressing.
5. Implicit/implied addressing.
Direct addressing
In this mode, the address of the operand is
explicitly specified within the instruction itself. All such instructions are 3
bytes long LDA and STA are two of the several instructions using direct
addressing.
For Example :LDA 4500H
Register addressing
In this mode, the data is in any one of the
general purpose registers and the register name is specified in the
instruction. This addressing mode is fast.
For Example: ADD C
Register Indirect
Addressing
In register indirect addressing mode, the
instruction specifies the name of the register in which the address of the data
is available. Here the data will be in memory and the address will be in the
register pair.
Example: MOV M,A - Stores the contents of accumulator into the memory
location pointed by HL register pair.
Immediate Addressing
Here the Instruction contains neither address
of the data nor register name that contains the data (or) Here the data is
given in the instructor itself. The 8-bit (or) 16-bit data follows the
instructor immediately. Immediate instruction is commonly used to provide
constant data type or for initializing the register.
(i)MVI A,10 (ii)ADI 10 (iii)LXI H,5000 (iv)CPI
05
Implicit / Implied
Addressing
There are some instructions that operate only
on one operand. Such instruction assume that the operand is in the Accumulator
and therefore require no address specification.
Example RLC, RRC and CMA
Assembly Language
The mnemonic (each manufacturer of a
microprocessor has devised a symbolic code for each instruction) for a
particular instruction consists of letters that represent the operation to be
performed by that instruction. Hence the computer manufactures have devised
English-like words represent the binary instructions of a machine, called
Assembly Language.
The mnemonics that can be written by
hand on paper and translated manually in hexadecimal code is called hand
assembly. Similarly, the mnemonics that can be written electronically on a
computer using a program and translated into binary code by using the program
is called assembler. Hence an assembler converts the program is assembly
language into machine language
Advantages/merits of
Assembly Language
·
Assembly language programs are easy to
understand than machine language.
·
Faster translation time.
·
Reduced Errors.
·
Change could be made easier and faster.
·
Small volume of data are to be
processed.
·
For industrial applications.
Disadvantages/Demerits
of Assembly Language
·
Assembly Language is different for
different machine (machine dependent)
·
It requires a translator to translate
assembly language to machine language
·
Many instructions are required to
achieve small tasks.
Assembly Language
Programming (ALP)
A program is a set of instructions
arranged in the specific sequence to do the specific code. The process of
writing the set of instructions which tells the microprocessor what to do is
called Programming." Assembly language operation codes (opcodes) are
easily remembered (MOV for move instructions, JMP for jump).
Assembly Language
Statement Format
Assembly
Language Programs are usually written in standard form. A typical assembly
language programming statement is divided into four parts called fields They
are
1. Label
2.
Operation code (Opcode)
3.
Operand
4
Comments
The
fields are separated by delimiters. The assembly language format of a
particular is shown below.
Label Opcode Operand Comment
LOOP: LXI H,2500 : Initiate the
HL register pair
1.
Label
--- This filed has a symbol or group of
symbols used to represent the address of other instructions. It is mainly used
in branching instructions. The assembler will assign memory address to the
labels in the label field. The length of the label should be written six
characters.
2.
Opcode --- The operation code field
usually contains the mnemonics instruction code. This field can never be empty.
The assembler will translate these opcode into its corresponding binary code.
3.
Operand --- This field contains the
name of registers, data or address of the memory location on which the
operation is to be performed.
4.
Comment ---This filed is used to
explain the function of the instruction.
Assembler Directives
The assembler directives are the
instructions to the assembler concerning the program to be assembled. They are
also called pseudo instructions. These instructions are neither translated into
machine code nor assigned any memory locations in the object file. Some of the
assembler directives are shown below.
ORG(Origin)
--This instructions specifies the address of following instructions to be
stored. For example, ORG 2500, instruct the assembler to store the program from
memory location 2500H.
END --This
instruction indicates the end of assembly language program.
EQU (Equate)
-- This instruction allow the user to assign numerical value to a symbolic name
DB (Define Byte)
-- This instruction instruct the assemble for reserving the successive memory
location.
DW (Define Word) -- This instruction instructs the
assembler for reserving two successive memory locations.
DS (Deſine Storage) --
This instruction instructs the assembler for reserving a block of memory
locations for data storage.
Instruction Format
An instruction is a binary pattern
entered through an input device to command the microprocessor to perform that
specific function. The 8085 instruction consists of one, two, and three byte
instructions,
One byte Instruction:
A one byte instruction includes the
Opcode and operand in the same byte
General Format: Opcode
Example: MOV D, A
This instruction copy the contents of
the accumulator into register.
Two Byte Instructions:
In a two byte instruction, the first
byte specifies the operation code(Opcode) and the second byte specifies the
operand.
General Format: Opcode
(Operand)
Example: MVI D,O5H
This instruction load on 8-bit data
(05H) in to the D register.
Three Byte Instructions:
In a three byte instruction, the first
byte specifies the Opcode and the second and third byte specifies the operand.
General Format: Opcode
operand1 operand2
Example: LDA 2050H
This instruction load the accumulator
with the content of memory location
2050H
Writing assembly level
programs
To write a program, one needs to draw up
a plan of logical thoughts. A given task should be broken down into small units
that can be built independently. This is called the modular design approach.
Writing a program is equivalent to
giving specific commands to the microprocessor in a sequence to perform a task.
The
programming process the following:
Step
1 : Read the problem carefully.
Step
2 : Break it down into small steps.
Step
3 : Represent these small steps in a possible sequence with a flow chart.
Step
4 : Translate each block of the flow chart into appropriate mnemonic
instruction.
Step
5: Translate mnemonics into machine code.
Step
6: Enter the machine code in memory and execute.
Step
7: In case of any error debug the program.
Instruction set of 8085
An instruction is a binary pattern
entered through an input device to command the microprocessor to perform that
specific function. The various instruction in 8085 are as follows:
·
Data transfer Instructions.
·
Arithmetic Instructions.
·
Branch Instructions.
·
Logical Instructions.
·
Machine Control Instructions.
Data-transfer
instructions with their meanings.
|
Opcode |
Operand |
Meaning |
Explanation |
|
MOV |
Rd, Sc |
Copy from the source (Sc) to the destination (Dt) |
This instruction copies the contents of the source
register into the destination register without any alteration. |
|
MVI |
Rd, data |
Move immediate 8-bit |
The 8-bit data is stored in the destination register or
memory. |
|
LDA |
16-bit address |
Load the Accumulator |
The contents of a memory location, specified by a 16-bit
address in the operand, are copied to the Accumulator. |
|
LDAX |
B/D Reg. pair |
Load the Accumulator indirect |
The contents of the designated register pair point to a
memory location. This instruction copies the contents of that memory location
into the Accumulator. |
|
LXI |
Reg. pair, 16-bit data |
Load the register pair immediate |
The instruction loads 16-bit data in the register pair
designated in the register or the memory. |
|
LHLD |
16-bit address |
Load H and L registers direct |
The instruction copies the contents of the memory location
pointed out by the address into register L and copies the contents of the
next memory location into register H. |
|
STA |
16-bit address |
16-bit address |
The contents of the Accumulator are copied into the memory
location specified by the operand. |
|
STAX |
16-bit address |
Store the Accumulator indirect |
The contents of the Accumulator are copied into the memory
location specified by the contents of the operand. |
|
SHLD |
16-bit address |
Store H and L registers direct |
The contents of register L are stored in the memory
location specified by the 16-bit address in the operand and the contents of H
register are stored into the next memory location by incrementing the
operand. |
|
XCHG |
None |
Exchange H and L with D and E |
The contents of register H are exchanged with the contents
of register D, and the contents of register L are exchanged with the contents
of register E. |
|
SPHL |
None |
Copy H and L registers to the stack pointer |
The instruction loads the contents of the H and L
registers into the stack pointer register. |
|
XTHL |
None |
Exchange H and L with top of stack |
The contents of the L register are exchanged with the
stack location pointed out by the contents of the stack pointer register. |
|
PUSH |
Reg. pair |
Push the register pair onto the stack |
The contents of the register pair designated in the
operand are copied onto the stack in the following sequence. |
|
POP |
Reg. pair |
Pop off stack to the register pair |
The contents of the memory location pointed out by the
stack pointer register are copied to the low-order register (C, E, L, status
flags) of the operand. |
|
OUT |
8-bit port address |
Output the data from the Accumulator to a port with 8 bit
address |
The contents of the Accumulator are copied into the I/O
port specified by the operand. |
|
IN |
8-bit port address |
Input data to Accumulator from a port with 8-bit address |
The contents of the input port designated in the operand
are read and loaded into the Accumulator. |
Arithmetic instructions
with their meanings.
|
Opcode |
Operand |
Meaning |
Explanation |
|
ADD |
R |
Add register or memory, to the Accumulator |
The contents of the register or memory are added to the
contents of the Accumulator and the result is stored in the Accumulator. |
|
ADC |
R |
Add register to the Accumulator with carry |
The contents of the register or memory & M the Carry
flag are added to the contents of the Accumulator and the result is stored in
the Accumulator. |
|
ADI |
8-bit data |
Add the immediate to the Accumulator |
The 8-bit data is added to the contents of the Accumulator
and the result is stored in the Accumulator. |
|
ACI |
8-bit data |
Add the immediate to the Accumulator with carry |
The 8-bit data and the Carry flag are added to the
contents of the Accumulator and the result is stored in the Accumulator. |
|
LXI |
Reg. pair, 16bit data |
Load the register pair immediate |
The instruction stores 16-bit data into the register pair
designated in the operand. |
|
DAD |
Reg. pair |
Add the register pair to H and L registers |
The 16-bit data of the specified register pair are added
to the contents of the HL register. |
|
SUB |
R |
Subtract the register or the memory from the Accumulator |
The contents of the register or the memory are subtracted
from the contents of the Accumulator, and the result is stored in the
Accumulator. |
|
SBB |
R |
Subtract the source and borrow from the Accumulator |
The contents of the register or the memory & M the
Borrow flag are subtracted from the contents of the Accumulator and the
result is placed in the Accumulator. |
|
SUI |
8-bit data |
Subtract the immediate from the Accumulator |
The 8-bit data is subtracted from the contents of the
Accumulator & the result is stored in the Accumulator. |
|
SBI |
8-bit data |
Subtract the immediate from the Accumulator with borrow |
The 8-bit data is subtracted to the contents of the Accumulator
and the result is stored in the Accumulator. |
|
INR |
R |
Increment the register or the memory by 1 |
The contents of the designated register or the memory are
incremented by 1 and their result is stored at the same place. |
|
INX |
R |
Increment register pair by 1 |
The contents of the designated register pair are
incremented by 1 and their result is stored at the same place. |
|
DCR |
R |
Decrement the register or the memory by 1 |
The contents of the designated register or memory are
decremented by 1 and their result is stored at the same place. |
|
DCX |
R |
Decrement the register pair by 1 |
The contents of the designated register pair are
decremented by 1 and their result is stored at the same place. |
|
DAA |
None |
Decimal adjust Accumulator |
The contents of the Accumulator are changed from a binary
value to two 4-bit BCD digits. |
Branching instructions
with their meanings.
|
Opcode |
Operand |
Meaning |
Explanation |
|||||||||||||||||||||||||||||
|
JMP |
16-bit
address |
Jump
unconditionally |
The program
sequence is transferred to the memory address
given in the operand. |
|||||||||||||||||||||||||||||
|
16-bit
address |
Jump
conditionally |
The program
sequence is transferred to the memory address
given in the operand based on the specified flag of the PSW. |
|||||||||||||||||||||||||||||
|
16-bit
address |
Unconditional
subroutine call |
The program
sequence is transferred to the memory address
given in the operand. Before
transferring, the address of the next instruction
after CALL is pushed onto the stack. |
|||||||||||||||||||||||||||||
|
RET |
None |
Return
from subroutine unconditionally |
The program
sequence is transferred from the subroutine to the calling program. |
|||||||||||||||||||||||||||||
|
None |
Return
from subroutine conditionally |
The program
sequence is transferred from the subroutine to the calling program based on
the specified flag of the PSW and the program execution begins at the new address. |
|||||||||||||||||||||||||||||
|
PCHL |
None |
Load
the program counter with HL contents |
The contents of
registers H & L are copied into the program counter. The contents of H are placed as the high-order Byte and the contents of L as
the low-order Byte. |
|||||||||||||||||||||||||||||
|
RST |
0-7 |
Restart |
The RST
instruction is used as software instructions in
a program to transfer the program
execution to one of the following eight locations.
The 8085 has
additionally 4 interrupts, which can generate RST instructions internally and
doesn’t require
any external hardware. Following are those instructions and their Restart
addresses
|
|||||||||||||||||||||||||||||
Logical instructions
with their meanings.
|
Opcode |
Operand |
Meaning |
Explanation |
|
CMP |
R |
Compare
the register or memory with the Accumulator |
The
contents of the operand (register or memory) are M compared with the contents
of the Accumulator. |
|
CPI |
8-bit
data |
Compare
immediate with the Accumulator |
The
second Byte data is compared with the contents of the Accumulator. |
|
ANA |
R |
Logical
AND register or memory with the Accumulator |
The
contents of the Accumulator are logically AND with M the contents of the
register or memory, and the result is placed in the Accumulator. |
|
ANI |
8-bit
data |
Logical
AND immediate with the Accumulator |
The
contents of the Accumulator are logically AND with the 8-bit data and the
result is placed in the Accumulator. |
|
XRA |
R |
Exclusive
OR register or memory with the Accumulator |
The
contents of the Accumulator are Exclusive OR with M the contents of the
register or memory, and the result is placed in the Accumulator. |
|
XRI |
8-bit
data |
Exclusive
OR immediate with the Accumulator |
The
contents of the Accumulator are Exclusive OR with the 8-bit data and the
result is placed in the Accumulator. |
|
ORA |
R |
Logical
OR register or memory with the Accumulator |
The
contents of the Accumulator are logically OR with M the contents of the
register or memory, and result is placed in the Accumulator. |
|
ORI |
8-bit
data |
Logical
OR immediate with the Accumulator |
The
contents of the Accumulator are logically OR with the 8-bit data and the
result is placed in the Accumulator. |
|
RLC |
None |
Rotate
the Accumulator left |
Each
binary bit of the Accumulator is rotated left by one position. Bit D7 is
placed in the position of D0 as well as in the Carry flag. CY is modified
according to bit D7. |
|
RRC |
None |
Rotate
the Accumulator right |
Each
binary bit of the Accumulator is rotated right by one position. Bit D0 is
placed in the position of D7 as well as in the Carry flag. CY is modified
according to bit D0. |
|
RAL |
None |
Rotate
the Accumulator left through carry |
Each
binary bit of the Accumulator is rotated left by one position through the
Carry flag. Bit D7 is placed in the Carry flag, and the Carry flag is placed
in the least significant position D0. CY is modified according to bit D7. |
|
RAR |
None |
Rotate
the Accumulator right through carry |
Each
binary bit of the Accumulator is rotated right by one position through the
Carry flag. Bit D0 is placed in the Carry flag, and the Carry flag is placed
in the most significant position D7. CY is modified according to bit D0. |
|
CMA |
None |
Complement
Accumulator |
The
contents of the Accumulator are complemented. No flags are affected. |
|
CMC |
None |
Complement
carry |
The
Carry flag is complemented. No other flags are affected. |
|
STC |
None |
Set
Carry |
Set
Carry |
Machine Control
instructions with their meanings.
|
Opcode |
Operand |
Meaning |
Explanation |
|
NOP |
None |
No
operation |
No
operation is performed, i.e., the instruction is fetched and decoded only. |
|
HLT |
None |
Halt
and enter wait state |
The
CPU finishes executing the current instruction and stops further execution.
An interrupt or reset is necessary to exit from the halt state. |
|
DI |
None |
Disable
interrupts |
The
interrupt enable flip-flop is reset and all the interrupts are disabled
except TRAP. |
|
EI |
None |
Enable
interrupts |
The
interrupt enable flip-flop is set and all the interrupts are enabled. |
|
RIM |
None |
Read
interrupt mask |
This
instruction is used to read the status of interrupts 7.5, 6.5, 5.5 and read
serial data input bit. |
|
SIM |
None |
Set
interrupt mask |
This
instruction is used to implement the interrupts 7.5, 6.5, 5.5 and serial data
output. |
RIM (READ Interrupt
Mask)
SIM (SET Interrupt
Mask)
Dynamic Debugging
The term Debugging means eliminating or
removing errors from the programs. The debugging process can be divided into
two parts :Static debugging and Dynamic Debugging .Static debugging is just a
visual inspection and done by checking the flowchart and machine code. After
the static debugging, if the program still does not produce the expected
output, one can attempt to debug the program by observing the execution of instructions.
This is called Dynamic Debugging,
Tools for Dynamic
Debugging
In a single-board micro computer, techniques
and tools commonly used in dynamic
debugging are:
1. Single-step
2 Register Examine
3. Breakpoint
Single-Step:
The single-step key on a keyboard allows
to execute one instruction at time, and to observe the results following each
instruction with the single Step by step technique and the following errors can
be found out
·
Incorrect addresses
·
Incorrect jump locations for loops
·
Incorrect data or missing codes.
Register Examine:
The Register Examine key allows to
examine the contents of the microprocessor register. After executing a block of
instructions, one can examine the register contents and compare these contents
with the expected outputs.
Breakpoint:
In the single-board computer, the
breakpoint facility is generally, a software
routine that allows to execute a program in sections. The breakpoint can be set
in your program by using RST instructions. The program will be executed until
the breakpoint. If the segment of the program is found correct, a second
breakpoint can be set at a subsequent memory address to debug the next segment
of the program. Using this technique the segment with errors can be isolated
and debugged with single-step facility. This technique can be used to check out
the timing loop, I/0 section and interrupts.
Counters
Counters are sequential circuits which
count the number of clock pulses. Counters are used to count number of clock
pulses, frequency division and mathematical manipulation. Counters are made up
of a set of Flip-flops. Designing a counter is a frequent programming
application. A counter is designed simply by loading an appropriate number into
one of the registers and using the INR (Increment by 1) or the DCR (Decrement
by 1)instructions.
A loop is established to update the
count and cach count is checked to determine whether it has reached the final
number, if not, the loop is repeated Counters are used primarily to keep track
of events.
Time Delay
·
Time delay is the timing between two
events.
·
A time delay is designed by loading a
register with a delay count and setting
up a loop to decrement the count until zero.
·
The delay is determined by the clock
period of the system and the time required to execute the instructions in the
loop.
·
A register is loaded, with a number,
depending on the time delay required, and then the register is decremented
until it reaches Zero by setting up a loop with a conditional Jump Instruction.
·
The loop causes the delay. depending
upon the clock period of the system.
·
Counter and time delays are important
techniques.
·
They are commonly used in applications
such as traffic signals, digital clocks. process control, and serial data
transfer.
·
The process of designing, counters and
time delays using software instructors is more flexible and less time consuming
than the design process using hardware
·
Time delays are important in setting up
reasonably accurate timing between two events.
The Time Delay is divided into three
types: They are:
1. Time Delay using one register
2 .Time Delay using register pair
3 .Time Delay using looping technique
Time
Delay using one register
MVI C,FF ;To execute this instruction the 8085 Mp
requires
7-T states(clock
periods)
LOOP:
DCR C ;Decrement C Here 4-T states
JNZ LOOP ;Jump
back to decrement C Here 10 or 7 states
Flow Diagram:
Calculation:
|
|
Where TL= Time delay in loop
T = System Clock period
N10 = equivalent decimal number of the hex number loaded in delay
register
Clock period T = 1/f =1/2 *10 -6 =
0.5 μs ( f=clock frequency = 2MHZ)
N10 = (FF)16 = (225)10 , loop T- states
= 4+10=14
TL=( T*Loop T states * N10)
=(0.5 *10 -6 *14*255)
= 1785 μs = 1.8 ms
TLA = TL-(3T –States *T)
=1785 μs -1.5 μs =1783.5 μs
To
calculate total delay as follows:
Total
delay = Time to execute outside loop instruction
+ Time to execute loop instruction
TD
= TO + TLA
= (7 *0.5 μs
) +1783.5 μs
= 1787 μs
Time Delay using a register
pair
The time delay is increased by setting a loop and using register pair with
a 16-bit number.The DCX instruction is used to decrement the register by 1.
Label Opcode Operand Comments T states
LXI B,2384 Load BC with 16-bit count 10
LOOP: DCX B Decrement
BC by 1 6
MOV A,C Place contents of C in A 4
ORA B OR B with C to set Zero
flag 4
JNZ LOOP if result not equal to 0 , 10/7
jump back to loop
·
The first instruction loads the BC
register pair with given data i.e B=23,C=84.
·
DCX decrements BC content by 1 i.e 2383.
·
DCX does not set the 0 flag and without
the test flags jump instruction cannot check the instruction.
·
So the next 2 instructions are used to
set zero flag.
·
OR instruction sets Zero flag only when
the contents of B and C are simultaneously zero.
·
Therefore the loop is repeated 2384
times.
Calculations:
Here,
N10 = 9092(i.e (2384)16
= (9092)10)
T – states = 24(DCX-6,MOV-4,ORA
-4,JNZ-10= 6+4+4+10 = 24)
T= 0.5 (because 2MHZ system)
TL= (0.5*24*9092) [TL=(T*T-states
*N10)]
= 109ms
Time Delay using Looping
technique:
A time delay similar to
that of a register pair can also be achieved by using two loops; one loop
inside other loop.
Mnemonics T states
MVI B, 38 7T
LOOP2:
MVI C, FF 7T
LOOP1:
DCR C 4T
JNZ LOOP1 10/7
T
DCR B 4T
JNZ LOOP2 10/7T
Calculations:
TL= TL1+TL2 (TL = Inner loop + Outer loop)
TL1= 1783.5 [Already discussed in
time delay using one register]
TL2
= (T*T-state * TL1)*N10
T = 0.5 ; T-States =21 ; TL1= 1783.5; N10=56
TL2= (0.5 *21 + 1783.5)*56
= (10.5+1783.5) *56
= 100.46ms
The
total delay should include execution time of first instruction (MVI B,38)
TD= TL1+TL2
+TLA
= 1783.5+100.46+7
= 1890.96ms
Stack
- The stack is a group of memory locations in R/W memory that is used for temporary storage of data during the execution of a program.
- Data has to be stored in the stack on a
last-in-first-out (LIFO) basis.
- Only two operations can be used on the
stack, the PUSH and POP operations.
- When the data is saved in the stack it
is called "PUSH on to stack”.
- When the data is read from the stack it
is called "POP from stack".
- The stack always has a unique location
known as the stack top.
- A special 16-bit register in the up
known as Stack Pointer (SP) used to hold the address of the stack top location.
- Any area of memory can be used as a
stack. The Microprocessor does not put only restriction on the location of the
stack in the memory.
- The maximum size of the stack depends on
the amount of memory available in the microcomputer.
- Most Microprocessor provides a stack
pointer for the maintenance of a stack in the memory Such a stack is also known
as an External Stack.
- Some up contains a few registers that
are used as stack. Such stacks are also known as internal stack
Programming with Stack
- The beginning of a stack is defined in
the program by using the instruction LXI SP,16-bit address which loads a 16-bit
address in the stack pointer register.
- Once the stack location is defined,
storing of data begins at the memory address that is one less than the address
in the stack pointer.
- For e.g IF Sp loaded with 4099, than the
storing of data begins at 4098 and continues in descending order (i.e
4098,4097,4096,…)
- Data bytes in the register pairs of the
up can be stored on the stack (two at a
time) in reverse order by using the instruction PUSH.
- Data bytes can be transferred from the
stack to respective registers by using the instructions POP.
- When the data is stored / pushed into
the stack the stack pointer decremented
by two.
- When the data is retrieved /popped from
the stack the stack point gets incremented by two.
- Because push and pop operations will
insert/delete two data types at a time.
- The address of the stack pointer
indicates the next two memory locations can be used for storage.
Example
POP PSW:
(content of stack pointer & flag register)
Contents
of stacks are copied into Acc & flag register.
-Higher
order bit firstà Flag
-Lower
order bit second àAcc
-Stack
pointer decrement by two
SPHL (load stack pointer with HL)
.
-The
contents of HL reg pair is loaded in the stack pointer.
Example
LXI SP, 6000 ; Here stack pointer is loaded with 6000
LXI H, 1234 ;This indicates that the memory space for
stack is
reserved
from 6000
PUSH H ;The starting location top first
location is 6000 and
can
move upwards that is descending
MVI, OF ;The data can be stored from 5999,
5998…..
Loop:
DCR H ;The contents of HL
(1234) is push into stack.
JNZ Loop ; A delay routine is performed at
last
POP H ;HL values are popped from
the stack
HLT ;Stop
Subroutine:
- A subroutine is a group of instructions
written separately from the main program a function that occurs repeatedly in
the main program.
- For example, if a time delay is required
between three successive events, three delays can be written in the main
program.
- To avoid repetition of the same delay
instructions, the subroutine technique is used
- Delay instructions are written once,
separately from the main program and are called by the main program when needed.
- 8085 up has two instructions to
implement subroutines
RET
- CALL is used to call a subroutine from
the main program.
- RET is used at the end of the subroutine
to return to the main program.
- When a subroutine is called, the content
of the program counter (which is the address of the instructions, following the
CALL instruction that is next to CALL instruction) is stored on the stack and
the program execution is transferred to the subroutine address.
- When the RET instruction is executed at
the end of the subroutine the address and the sequence of execution is resumed
in the main program.
Example
conversion
of ASCII to HEX
example
for CALL and RET instructions.
MVI A,
35 ;Aß35
CALL HEX
;CALL the subroutine HEX
STA 8000
;After return store Acc at 8000
HLT ;STOP
HEX:
SUI 30 ;subroutine HEX: Aß
A-30
CPI OA ;compare Acc with 10
RC ;Return to main program if
cy=1
SUI 07 ;if cy=0 AßA-07
RET ;return to calling program.
Debugging Counter and
Time Delay Program:
Debugging Counter and Time-Delay Programs
·
The Static and Dynamic debugging
techniques can be used to check errors in a counter program
· The following is a list of common errors in programs.
· Errors in counting T-States in a delay loop.
- Errors in recognizing how many times a
loop is repeated.
- Failure to convert a delay count from
Decimal to hexadecimal equivalent
- Specify a wrong jump location.
- Failure to set a flag.
- Using a wrong jump instruction.
- Failure to display either the first or
last count.
- Failure to provide a delay between the
last and last-but-one count.
Example
MVI
A, 64
DSPLAY:
OUT PORTI
LOOP2
: MVI B, 10
LOOP1: LXI D. DELAY
DCX
D
NOP
NOP
MOV
A, D
ORA
E
JNZ
LOOP1
DCR
B
JZ
LOOP2
DCR
A
CPI
00
JNZ DISPLAY
·
The above program is designed to count
from 100 to 0 Hex continuously, with a l-second delay belween each count.
•
The delay is setup by using two loops-a loop within a loop.
•
The inner loop is used to provide 100 ms delay.
•
The outer loop is used to repeat inner loop for 10 times to provide second
total delay
•
The clock period of the system is 330 ns.
Debugging the given
program
The
given program consists of several errors:
1.
The label Loop is not at the correct location So alter Loopl in a correct
place.
2.
Calculate the delay count accurately and move to DE.
3.
The 8th instruction MOV A. D is wrong and it will spoil the program
because the count 64 will be over write by the value of D if the instruction is
executed.
4.
The location of Loop2 is not correct and the 12th instruction JZ Loop2
is not a correct condition.
5.
The 14th instruction CPI is not necessary. There is no need for this
instruction.
6.
The given Delay calculations are wrong and calculate the new Delay calculations
for the new program.
After
Debugging T-States
MVI H, 64 7
DISPLAY:
OUT PORTI 10
MVI B, OA 7
Loop2: LXI D, DELAY 10
Loopl: DCX D 6
NOP
4
NOP
4
MOV A, E 4
ORA D 4
JNZ Loop1 10/7
DCR B 4
JNZ Loop2 10/7
DCR H 4
JNZ DISPLAY 10/7
DELAY CALCULATIONS:
Conditional Call and Return Instructions
CALL (Unconditional
Call)
CALL
address (unconditional call/subroutines)
• Before call instruction is executed the
address of the next to CALL,
instruction is loaded in the stack and execution continues.
·
When a return instruction is executed
the address which was saved in stack is passed and execution starts from that
address.
·
CALL instruction is a three byte
instruction. The first byte specifies the Opcode, the second and third gives
the 16-bit starting address of the instruction
Conditional Call
Instruction Flags checked
CC
address Cy=1
CNC
address Cy=0
CZ
address Z=1
CNZ
address Z=0
CP
address S=0
CM
address S=1
CPE
address P=1
CPO
address P=0
Unconditional Return
This instruction pops the return address
from the stack and loads Program counter with this return address. Return to
the calling program after completing the subroutine.
General
Format: RET
Example
:RET
Conditional Return
Instruction Flags Checked
RC
address Cy=1
RNC
dress Cy=0
RZ
address Z=1
RNZ
address Z=0
RP
address S=0
RM
address S=1
RPE
address P=1
RPO
address P=0
Restart Instruction
(RST:0,1,2,3,4,5,6,7)
The restart instruction is an unconditional
call instruction. This instruction generally used in interrupts and software
instructions in a program.
Instruction
Effect
Branching Binary code Opcode
RST
0 CALL
0000 0000 1100 0111 C7
RST
1 CALL 0008 0008
1100 1111 CF
RST
2 CALL 0010 0010 1101 0111 D7
RST
3 CALL 0018 0018 1101 1111 DF
RST
4 CALL 0020 0020
1110 0111 E7
RST
5 CALL 0028 0028
1110 1111 EF
RST
6 CALL 0030 0030 1111 0111 F7
RST
7 CALL 0038
0038 1111 1111 FF
•
The 8085 has 8 restart instructions RST 0,RST
1, RST 2, RST 3,RST 4,
RST 5, RST 6, RST 7
•
These instructions represent efficient
ways to call frequently used subroutine.
•
A restart (RST) has the same effect as
CALL.
•
On the execution of RST 0. the program
branches to 0000 and the instructions are executed from there till it reaches
the RET statement.
•
A RST is special kind of CALL because it
branches to a predetermined address.
•
The RST Instruction is 1-byte long
whereas the CALL Instruction 3-byte long.
•
Therefore a RST instruction is an
efficient way to call frequently used subroutines
•
RST 0 to RST 7, are called software
Interrupts because they are instructions.
•
The 8085 has hardware interrupts namely
TRAP, RST 7.5, RST 6.5,
RST 5.5,INT.
Hexadecimal Counter:
•
·A counter which counts continuously in
hexadecimal form with appropriate time delay between each count is called
"Hexa decimal counter"
•
Let us design a Hexadecimal counter,
which can count continuously in hexadecimal from FF to 00 in a system with a 0.5
μs clock period.
•
We will use 'C' register to set up a l
millisecond (ms) delay between each count and display numbers at one of the
output port.
Problem Analysis
To design a Hexadecimal counter we have
to do two things:
1.
To set up a continuous down counter.
2.
To design a given delay between two counts.
Setup Counter
•
The Hexadecimal counter is set up by
loading a register with an appropriate starting no. and decrementing it until
it becomes zero.
•
After zero count, register goes back to
FF because decrementing zero results in a (-1) which is FF in 2's complement.
(0)16 = 0000 0000
1111 1111
(F) (F)
FLOW DIAGRAM
Program:
Mnemonics Comments
MVI B, 00 B is a counter
NEXT: DCR B Decrement B
MVI C, 8CH Cß
Delay count
DELAY: DCR C Decrement Delay count
JNZ DELAY Jump to Delay
MOV A, B if #0
OUT PORT# move B to A and display the value
through Port X
JMP NEXT Go To Next
Modulo Ten Counter
•
The Modulus of a counter refers to how
many counts it will go through before it repeats.
•
The modulus of a counter is the number
of output states it has.
•
If counters go through ten different
states (0 to 9) then it is called as MOD 10 counter.
•
Let us Design a MOD - 10 counter which
can count from 0 to 9 with a second delay between each count.
•
At the count of 9,the counter should
reset itself to 0 and repeat the sequence continuously.
•
Use HL register pair to set up the
delay, and display each count at one of the output ports.
•
Assume clock frequency of micro computer
is 1 MHz.
FLOW CHART:
Program:
Mnemonics
START:
MVI B, 00 H
MOV A, B
DSPLAY:
OUT PORT #
LXI H, 16-bit
LOOP: DCX H
MOV A, L
ORA H
JNZ LOOP
INR B
MOV A, B
CPI 0A H
JNZ DSPLAY
JZ START
Program description:
•
The counter is setup by loading the
register with the appropriate numbers and incrementing it until reaches the
digit 9.
•
When counter reaches the find count, it
is reset to zero.
•
The 1-second delay between the each
count is set up by using a register pair.
•
In the program, the instructions ORA is
used to set the zero fag.
•
This ORA instructions is used to check
whether HC has reached Zero.
•
The CPI OA checks and compares the
content of B register in every cycle.
•
When register B reaches the number OA,
the program sequence is redirected to reset the counter without displaying the
contents.
Pulse timings for
flashing lights
Let us write a program to generate a
continuous square wave with the period of 500 μs. Assume the system clock
period is 325 ns and use bit D0 to output the square wave.
•
For the above program, period of square
wave is 500 μs, therefore the pulse should be on (logic 1) for 250 μs and off(
logic 0) for 250 μs
•
Alternate pattern of 0/1 bits can be
provided by loading the accumulator with AA(1010 1010) and rotating the pattern
once through each delay loop
•
Bit D0 of the output port is
used to provide logic 0 & 1
•
All other bits can be marked by AND the
Acc with the byte 01.
Program Description:
•
Register D is loaded with AA (bit
pattern)
•
D is moved to Acc and rotated left and again stored in D (i.e)
1010 1010 à 0101 0101(RLC)
•
ANI ANDs Accumulator with 01 to mask all
bits expect D0.
Acc
= 0101 0101
01 =
0000 0001
(AND) = 0000 0001 à
Accumulator
•
D0=1 provides high pulse during the
first 250 μs because of the delay.
•
In the next cycle of the loop, D0=0
because of RLC and the output pulse stays low for the next 250 μs.
PROGRAM:
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