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Microprocessor – Arrangement of 8085 legs.
The 8085 Microprocessor – Pinouts and Operation
The 8085 microprocessor is an 8-bit microprocessor developed by Intel in 1976. It is one of the first popular microprocessors and is widely used in embedded applications and education. Below is detailed information about the pinouts of the 8085 microprocessor and how it operates.
Hello engineers, hope you are all well. In this article, we will discuss the basic architecture of 8085 microprocessor and 8085 microprocessor pins and how it works. So let’s start with the introduction to 8085 microprocessor.
Before we actually get into the architecture and pinouts of the 8085 microprocessor, we can check out its history and basic features first.
Introduction to 8085 microprocessor
The 8085 is an 8-bit microprocessor first introduced in March 1976 by Intel. It uses N-type metal-oxide-semiconductor (N-MOS) technology. It is a 40-pin microprocessor packaged in a DIP (Dual Inline Package) package manufactured on a single LSI chip. The 8085 uses a single +5V DC supply for its operation. It requires a clock speed of about 3 MHz.
It consists of three main parts, the computation and logic unit, the synchronization and control unit, and a number of registers for different purposes.
Intel 8085A has a clock cycle of 320 ns (nanoseconds). The backoff cycle time of Intel 8085 A-2 is 200 ns. It has 80 basic instructions and 246 opcodes.
The 8085 was not the first microprocessor to be introduced. The world’s first microprocessor was the Intel 4004, a complete microprocessor on a chip released in March 1971, and used advanced silicon-gate technology.
Let’s understand the basic architecture of this microprocessor through the block diagram given here.
8085 microprocessor architecture .
We will understand and discuss the physical structure and pinouts of 8085 microprocessor after discussing its architectural structure.
Starting with Interrupt Control, when any hardware interrupt signal is received at the microprocessor, it stops the current execution. And now, the program control is transferred to a sub-routine with the help of CALL signal and after executing that sub-routine with RET signal, the program control is again returned to the main program from where it had stopped.
There are 5 interrupt handles, INTR (lowest priority), RST 7.5, RST 6.5, RST 5.5, TRAP. We will study all these PINs in more detail later in our study of 8085 Pinouts.
The processor recognizes an interrupt and sends an acknowledgement to the requested peripheral via INTA.
An 8-bit internal Data Bus is connected between the IC and the I/O devices. This is used to transfer 8 bits of data at a time.
Registers are temporary storage locations for temporary storage of data and manipulation of data and instructions by the microprocessor. Data remains in registers until used and then cleared. For example, you may remember the use of variables in programming.
Registers are of different types mentioned below:
6 shared, 8-bit registers, B, C, D, E, H and L.
A 16-bit stack pointer, SP
A 16-bit program counter, PC
Instruction register
Temporary register
The synthesizer is an 8-bit register, which is connected to the ALU (Arithmetic and Logic Unit). It is used to hold temporary data and results of logical operations performed by the ALU during execution. After the operation, the final result of an arithmetic or logical operation is also stored in the synthesizer.
The output of the synthesizer is connected to the ALU.
ALU (Arithmetic and Logic Unit): ALU is the heart of a microprocessor. This unit is responsible for all the arithmetic and logical operations. ALU operations like addition, subtraction, increment, decrement or AND, OR, NOR, NOT on 8 bits of data can be performed by ALU.
The results are stored in the aggregator.
Temporary Register: This is also an 8-bit register used to store temporary data of the ALU. This register can only be accessed by our microprocessor. They are of two types W and Z.
Flag Register: This is an 8-bit register with five 1-bit flip-flops in it. After the result is stored in the synthesizer, the 5 status flags are set or reset depending on the type of result of the operation.
Carry Flag (CY): It occupies bit 0, the least significant bit. If the arithmetic operation results in a carry then the Carry Flag is set, otherwise it is reset.
Parity Flag (P): It occupies the 2nd bit and if the number of 1s in the main memory is even then it is set, otherwise it is reset and is called odd parity. and vice versa.
Auxiliary Carry Flag (AC): It is located at the 4th bit. Auxiliary carry means that the carry created by the 3rd bit is carried to the 4th bit.
Zero Flag (Z): It is located in the 6th bit of the flag register. It is set when the result is 0, otherwise it is reset. It is also useful to determine whether two operands are equal or not.
Sign Flag (S): It occupies the 7th bit (most significant bit). Checks whether the result is positive or negative.
If a flip-flop for a particular flag is set, it indicates 1. When it is reset, it indicates 0.
General Purpose Registers: We have 6 general purpose registers in our 8085 microprocessor. Those 6 general purpose registers are B, C, D, E, H, L. Each register can hold 8 bits of data and if they work in pairs like BC, DE, HL, then there are 16 bits.
Instruction Register: This is an 8 bit register and it stores the instructions. It holds the instructions that need to be executed. The instructions that are fetched from the memory are stored in this register of our microprocessor.
Instruction Decoder: It is used to decode instructions from the instruction register.
Stack Pointer: This is a 16-bit special function register that stores the address of an instruction or data that is required for later processing and is stored on the stack. The stack pointer is a part of memory (part of RAM).
Program Counter: This is a 16-bit special function register used to store the memory location address of the next instruction to be executed.
It is incremented so that when one execution is finished it points to the address of the next instruction in the program.
Timing Controller: This is the part of the CPU that generates timing pulses and control signals for execution. It also controls the flow of data between the CPU and the peripherals. It controls the adjustment between the microprocessor and the peripherals connected to it.
The Intel 8085 is a compact microprocessor with the following connector pins:
– Pin 1- Pin 2(x1,x2)– The microprocessor requires a clock signal to function properly with other peripherals connected to it. Pins 1 and 2 are used to connect a crystal oscillator to receive the clock signal. In the case of 8085, we use a 6mhz crystal.
– PIN3 RESET OUT- To reset all the functions of other devices connected to the microprocessor.
– PIN4 SOD– SOD is the serial data output pin for sequential data output.
– PIN5 SID– SID is the serial data input pin, used for sequential data input. The data on this pin is loaded into the synthesizer on the 7th bit every time a RIM instruction is executed.
– PIN6 TRAP– TRAP is the highest priority interrupt. This is an unmaskable interrupt, i.e. it is not affected by any mask or other interrupt.
– PIN7-9 RST5.5, RST6.5, RST7.5- These are shieldable interrupts, of which RST7.5 has the highest priority.
– PIN10 INTR-INTR is the lowest priority shieldable interrupt. This is an input interrupt request from a peripheral.
– PIN11 INTA’- INTA is Interrupt Acknowledge. It is used instead of RD in the instruction cycle after the INTR request is accepted. It is not an interrupt and is used by the microprocessor to send an acknowledgement.
– PIN12 – 19( AD0-AD7)– These are combined address + data pins since the 8085 uses 16-bit address pins and can only hold 8-bit data, so these pins are general-purpose and bidirectional. Low-order address pins. They are the pins that are combined multiplexed address and data bus timing.
– PIN20 VSS– Ground only. Connect to negative.
– PIN21-28(A8-A15)– These are address pins and can only go one way. High order address bus.
– PIN29 & PIN33 (S0 and S1)– These are status pins. They indicate the operation being performed. A small status pin table can show how it works.
– PIN3 RESET OUT- To reset all the functions of other devices connected to the microprocessor.
– PIN4 SOD– SOD is the serial data output pin for sequential data output.
– PIN5 SID– SID is the serial data input pin, used for sequential data input. The data on this pin is loaded into the synthesizer on the 7th bit every time a RIM instruction is executed.
– PIN6 TRAP– TRAP is the highest priority interrupt. This is an unmaskable interrupt, i.e. it is not affected by any mask or other interrupt.
– PIN7-9 RST5.5, RST6.5, RST7.5- These are shieldable interrupts, of which RST7.5 has the highest priority.
– PIN10 INTR-INTR is the lowest priority shieldable interrupt. This is an input interrupt request from a peripheral.
– PIN11 INTA’- INTA is Interrupt Acknowledge. It is used instead of RD in the instruction cycle after the INTR request is accepted. It is not an interrupt and is used by the microprocessor to send an acknowledgement.
– PIN12 – 19( AD0-AD7)– These are combined address + data pins since the 8085 uses 16-bit address pins and can only hold 8-bit data, so these pins are general-purpose and bidirectional. Low-order address pins. They are the pins that are combined multiplexed address and data bus timing.
– PIN20 VSS– Ground only. Connect to negative.
– PIN21-28(A8-A15)– These are address pins and can only go one way. High order address bus.
– PIN29 & PIN33 (S0 and S1)– These are status pins. They indicate the operation being performed. A small status pin table can show how it works.
13. PIN32 RD’– The PIN32 read signal is an active low signal. That means when it is low, it is active. It belongs to the control signal category.
14. PIN31 WR’– Write signal, active low. Control signal.
15. PIN34 IO/M’– This signal indicates whether the operation is a memory operation or an I/O operation. When IO/M is HIGH(1) it is an I/O operation and when it is LOW(0) it is a memory operation.
16. PIN35 READY– Also a control signal. This indicates that the peripheral is ready to receive or write data. When this pin is high during any read or write cycle.
17. PIN36 RESET IN– When this pin goes high(1), it resets the internal process of the processor. The program counter is set to 0 and the CPU remains in the reset state until this pin goes high.
19. PIN30 ALE– Enable Address Latch, it is HIGH(1) when pins AD0-AD7 are used as address lines, otherwise 0. When ALE is 1, then the current line number will be the address and when 0, then the current number on the multi-select line will be the data.
20. PIN37 CLK OUT-Clock out, clock output pin to control the clock of the remaining system.
21. PIN38 & 39– HOLD(PIN 39) is the DMA signal pin. It is the signal that another MASTER is requesting to use the Address and Data Buses for DMA access. PIN38 is the HLDA hold acknowledgement pin, accepting the DMA request. HLDA is high when indicating that the CPU has received the Hold request and will provide the Buses in the next clock cycle.
HLDA is low when request is rejected.
22. PIN40 VCC– The VCC pin is for +5v DC input to the IC to have enough power supply to operate.
The 8085 is no longer used in computers used in this advanced era, but is still useful in automation systems (not so much) and measurement systems. Also, it has now been moved to the toy industry.
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Thank you.