Hardware Architecture of 8085 MicroprocessorInstruction with 6 T-states in Opcode Fetch Cycle : PCHL , DCX H Memory Read Machine Cycle: In this machine cycle , 8085 microprocessor

Prepared by Mr.S.Sayeekumar , AP/ EEE , RMD ENGINEERING COLLEGE

Hardware Architecture of 8085 Microprocessor :

There are totally six programmer accessible registers namely B,C,D,E,H,L that can either function as single 8-

bit register (such as B,C,D,E,H,L) or as 16-bit register pair such as BC , DE, HL. In case of register pair the higher order byte is stored in first register (for example B-register in BC register pair) and lower order byte is stored in second register (for example C-register in BC register pair). The HL register additionally functions as MEMORY POINTER or DATA POINTER in MEMORY BASED DATA MOVEMENT INSTRUCTIONS.

The 8-bit registers W and Z are used for TEMPORARY DATA STORAGE when data is (i) moved between registers (ii) Moved from internal registers to external data bus and vice-versa.

The 16-bit STACK POINTER(SP) is used to hold the address of TOP of STACK residing in external RAM. TOP OF STACK is a STACK LOCATION where a RECENT PUSH OPERATION occurred. The type of STACK implemented is LIFO(LAST IN FIRST OUT) , wherein the the element that is inserted into the LAST STACK location will be the


FIRST element to be removed. Two types of operations associated with STACK : (i) PUSH(INSERT) (ii) POP (REMOVE). The SP gets decremented by 2 for every PUSH operation and incremented by 2 for every POP operation. Default value of SP = FFFFH

The 16-bit PROGRAM COUNTER (PC) is used to hold the ADDRESS OF NEXT INSTRUCTION TO BE EXECUTED. It increments or decrements by :

1 for 1-byte instruction 2 for 2-byte instruction 3 for 3-byte instruction Default Value of PC = 0000H The Incrementer/Decrementer Address Latch is used to hold the INCREMENTED or DECREMENTED contents of PC and SP. The 8-bit BUFFERS (Lower order MULTIPLEXED Address and Data Bus Buffer[AD7-AD0] and Higher order Address Bus Buffer[A15-A8]) acts as a DATA COMMUNICATION INTERFACE between INTERNAL DATA BUS and EXTERNAL DATA BUS. The address bus (A15-A0) carries the content of PC and SP. The data bus (D7-D0) carries the content of BC , DE , PSW , HL register pairs. The 8-bit INSTRUCTION REGISTER is used to hold the OPCODE (HEXADECIMAL EQUIVALENT of an INSTRUCTION). The INSTRUCTION DECODER is used for enabling a PARTICULAR INTERNAL COMPONENT of 8085 microprocessor based upon the instruction received. The MACHINE CYCLE ENCODER is used for providing information about the CURRENTLY EXECUTING MACHINE CYCLE of an INSTRUCTION EXECUTION. The 8-bit ALU is used to perform the following ARITHMETIC and LOGICAL OPERATIONS : ARITHMETIC OPERATIONS : 8-bit and 16-bit ADDITION with and without CARRY 8-bit and 16-bit SUBTRACTION with and without BORROW 8-bit and 16-bit INCREMENT operation 8-bit and 16-bit DECREMENT operation 8-bit BINARY to BCD CONVERSION LOGICAL OPERATIONS : 8-bit AND , OR , NOT , XOR operations 1-bit SET and RESET operation 8-bit ROTATE LEFT and RIGHT operations with and without CARRY. The components of ALU are : (i) ACCUMULATOR (ii) TEMP (iii) 8-bit ALU (iv) FLAG ACCUMULATOR :

8-bit REGISTER that acts as SOURCE or DESTINATION REGISTER in case of DATA TRANSFER INSTRUCTIONS

8-bit REGISTER that acts as SOURCE AND/OR DESTINATION REGISTER in case of DATA MANIPULATION (or ARITHMETIC and LOGICAL) INSTRUCTIONS.

First SOURCE operand in case of DATA MANIPULATION INSTRUCTIONS. TEMP : 8-bit REGISTER that acts as SECOND SOURCE operand in case of DATA MANIPULATION INSTRUCTIONS.


FLAG REGISTER:

D7 D6 D4 D2 D0

S Z AC P CY

Positive Number or Negative Number in the ACCUMULATOR (Sign bit [S]) Even Parity or Odd Parity in ACCUMULATOR (Parity bit [P]) Carry or borrow occurring at HIGHER ORDER NIBBLE of ACCUMULATOR (Carry bit or Overflow bit [CY]) Carry or borrow occurring at LOWER ORDER NIBBLE of ACCUMULATOR(Auxiliary Carry bit[AC]) Zero or Non-zero value in ACCUMULATOR(Zero bit). Combination of ACCUMULATOR and FLAG REGISTER is called PSW (Program Status Word).

ACCUMULATOR(8-BITS) FLAG(5-BITS)

FIG. PSW.

The TIMING and CONTROL circuitry is used for GENERATING TIMING , CONTROL and STATUS signals that is required for operation of INTERNAL and EXTERNAL CIRCUITRIES of 8085 microprocessor IN SYNCHRONISM

The SERIAL CONTROL CIRCUITRY is used for SHIFTING the contents into and out-of of ACCUMULATOR and an EXTERNAL REMOTE PERIPHERAL DEVICE as SEQUENCE of 1-BITs.

The INTERRUPT CONTROL CIRCUITRY is used for receiving EDGE AND/OR LEVEL TRIGGERED INTERRUPT SIGNALS from EXTERNAL PERIPHERAL DEVICES and accordingly SEND an ACKNOWLEDGEMENT to the interrupting device when the CPU branches to the corresponding Interrupt Vector Address. 8085 supports 5 hardware interrupts and 8 software interrupts.


Pin diagram of 8085 Microprocessor :

S.No PIN Number Description

POWER SUPPLY AND FREQUENCY SIGNALS

1. VCC Connected to +5V.

2. VSS Connected to GROUND

3. X1 and X2 LC circuit or RC circuit or Crystal connected between these two pins. Provides the INPUT signal for generating CLOCK signal.

4. CLK OUT Frequency of CLOCK output signal = 0.5 × Frequency of input signal.

ADDRESS AND DATA BUS

5. AD0-AD7 Lower order 8-bit multiplexed address and data bus Acts as Address Bus during T1 state of a machine cycle and Data Bus during T2 and

T3 states of a machine cycle. When acting as address bus , it carries lower order byte of a memory location or

the 8-bit address of a I/O port.

6. A8-A15 Higher order address bus Acts as Address bus throughout a machine cycle.

When acting as address bus , it carries higher order byte of a memory location or the 8-bit address of I/O port


CONTROL AND STATUS SIGNALS

7. ALE ADDRESS LATCH ENABLE SIGNAL Enables an external latch for demultiplexing the AD0-AD7 bus into A0-A7 and D0-

D7. Active during T1 state of each machine cycle

8. 𝑅𝐷̅̅ ̅̅ and 𝑊𝑅̅̅ ̅̅ ̅ Active low signals When RD signal is activated (ie.RD = 0) , the microprocessor reads a 8-bit data

from a selected memory location or Input port When WR signal is activated (i.e. WR = 0) , the microprocessor writes a 8-bit data

into a selected memory location or output port. Active during T2 and T3 states of each machine cycle.

9. IO/�̅� , S0 , S1

IO/�̅� signal indicates whether 8085 microprocessor is currently accessing a memory location or I/O port. IO/�̅� = 0 means 8085 is accessing a memory

location and IO/�̅� = 1 means 8085 is accessing a I/O port. IO/�̅� together with status signals S1 and S0 indicate what is the current machine

cycle executed by 8085 microprocessor.

Machine Cycle Status Signals

IO/�̅� S1 S0

Bus Idle or No Operation

0 0 0

Memory Write 0 0 1

I/O Write 1 0 1

Memory Read 0 1 0

I/O Read 1 1 0

INTR Acknowledge

1 1 1

INTERRUPT SIGNALS

9. RST 5.5 – RST 7.5 , TRAP , INTR , 𝐼𝑁𝑇𝐴̅̅ ̅̅ ̅̅ ̅

8085 microprocessor supports 5 hardware interrupts. Activating any one pin (TRAP or RST 7.5 or RST 6.5 or RST 5.5 or INTR) causes the

microprocessor to complete its CURRENT INSTRUCTION EXECUTION. Activation of INTA(INTerrupt Acknowledgement) signal by 8085 microprocessor

indicates it has accepted the INTR interrupt from the requesting device.

SERIAL I/O SIGNALS

10. SID , SOD SID refers to SERIAL INPUT DATA PIN CONTENT IS FED INTO THE ACCUMULATOR FROM AN EXTERNAL DEVICE

THROUGH ITS MOST SIGNIFICANT BIT. SOD refers to SERIAL OUTPUT DATA PIN

CONTENT IS FED OUT FROM THE ACCUMULATOR TO AN EXTERNAL DEVICE THROUGH ITS MOST SIGNIFICANT BIT.

DMA (DIRECT MEMORY ACCESS) SIGNALS

11. HOLD , 𝐻𝐿𝐷𝐴̅̅ ̅̅ ̅̅ ̅̅

HOLD SIGNAL indicates that another microcontroller or microprocessor is requesting the 8085 microprocessor to use its address bus , data bus and control

bus. HLDA SIGNAL indicates the 8085 microprocessor has given permission to usage of

its buses to the requesting microcontroller or microprocessor

RESET SIGNALS


12. 𝑅𝐸𝑆𝐸𝑇 𝐼𝑁̅̅ ̅̅ ̅̅ ̅̅ ̅̅ ̅̅ ̅ , RESET OUT

Activating the RESET IN SIGNAL does the following :

Sets the PC value to 0000H

Clears the INTE (INTERRUPT ENABLE) flag (i.e. Interrupts are disabled)

Clears the HLDA flip-flop (i.e. DMA operation is discontinued)

Tri-states the address bus , data bus and control bus

Assigns random values to data registers. Activation of RESET OUT signal indicates the following :

Reset operation is going inside the processor

Used for resetting the devices connected externally to 8085 microprocessor

Timing Diagrams of 8085 Microprocessor :

Instruction Cycle : Fetching , Decoding and Execution of a single 8085 instruction; Group of 1 to 5 machine cycles constitute an Instruction cycle. Machine Cycle : A Memory or I/O read and write operation ; Group of 3 to 6 T-states constitute a Machine cycle. T-state or Clock period or clock cycle : A clock pulse consisting of falling edge followed by rising edge. T-state is the time interval between TWO NEGATIVE or FALLING CLOCK EDGES. Different types of machine cycles are as follows: Opcode or Instruction Fetch Machine Cycle :

In this machine cycle , 8085 MICROPROCESSOR finds the nature of instruction to be executed.

During T1 state , the 8085 microprocessor places the contents of program counter on the address bus.

The Higher Order Byte of PC placed in A15-A8 bus and Lower Order Byte of PC placed in AD7-AD0 bus. The ALE signal is activated which separates the AD7-AD0 bus by enabling an external latch for holding the lower order byte of PC.

During T2 state , the RD signal enables the memory location present in A15-A0 bus.

During T3 state , the opcode from the addressed memory location is placed in D7-D0 bus and the RD signal is disabled.


During T4 state , the opcode is decoded by the instruction decoder to decide whether to enter another T5 , T6-states or to enter T1 state of next machine cycle.

During T5 and T6 states , 8085 microprocessor performs STACK WRITE , INTERNAL 16-bit and CONDITIONAL RETURN operations depending upon the type of instruction.

Instruction with 4 T-states in Opcode Fetch Cycle : MOV A , B ; ADD B Instruction with 6 T-states in Opcode Fetch Cycle : PCHL , DCX H Memory Read Machine Cycle :

In this machine cycle , 8085 microprocessor reads the data from a external RAM or ROM location.

During T1 state , the 8085 microprocessor places the contents of PC or SP or General Purpose Register Pair on the address bus. The Higher Order Byte of PC or SP or the register pair placed in A15-A8 bus and Lower Order Byte of PC or SP or the register pair placed in AD7-AD0 bus. The ALE signal is activated which separates the AD7-AD0 bus by enabling an external latch for holding the lower order byte of PC or SP or register pair.

During T2 state , the RD signal enables the memory location present in A15-A0 bus and 8-bit data is placed in D7-D0 bus.

During T3 state , the 8-bit in D7-D0 bus is placed into a 8-bit register and the RD signal is disabled.


Memory Write Machine Cycle :

In this machine cycle , 8085 microprocessor writes the data into a external RAM or ROM location.

During T1 state , the 8085 microprocessor places the contents of SP or General Purpose Register Pair on the address bus. The Higher Order Byte of SP or the register pair placed in A15-A8 bus and Lower Order Byte of SP or the register pair placed in AD7-AD0 bus. The ALE signal is activated which separates the AD7-AD0 bus by enabling an external latch for holding the lower order byte of SP or register pair.

During T2 state , the WR signal enables the memory location present in A15-A0 bus and data in the D7-D0 bus.

During T3 state , the WR signal is disable the memory location , thereby terminating the write cycle. I/O Read Machine Cycle :

In this machine cycle , 8085 microprocessor reads the data from an external input port.


During T1 state , the 8085 microprocessor places the input port address on A15-A0 bus.

During T2 state , the RD signal enables the Input Port present in A15-A8 bus or A7-A0 bus and 8-bit data is placed in D7-D0 bus.

During T3 state , the 8-bit in D7-D0 bus is placed into a 8-bit register and the RD signal is disabled. I/O Write Machine Cycle:

In this machine cycle , 8085 microprocessor writes the data into an external output port.

During T1 state , the 8085 microprocessor places the input port address on A15-A0 bus. During T2 state , the WR signal enables the Output Port present in A15-A8 bus or A7-A0 bus and 8-bit data is placed in D7-D0 bus. During T3 state , the 8-bit in D7-D0 bus is placed into the addressed output port and the WR signal is disabled.

Interrupt Acknowledgement Cycle :

In this machine cycle , 8085 microprocessor responds to an interrupt request sent by a peripheral device .

The peripheral device places a RST or CALL instruction in the D7-D0 bus.

In case if the received opcode is that of CALL instruction , three machine cycles are taken. One cycle for receiving the opcode , second cycle and thire cycles for receiving the branching address. During these machine cycles , the value of PC is not incremented.

During 4th and 5th machine cycles , the current contents of PC are written into the TOP of STACK pointed by the SP.

Then the 8085 microprocessor places the branching address onto PC.

In case if the received opcode is that of RST instruction (Software Interrupt) , execution is similar to that of CALL instruction , except 4th and 5th machine cycles does not occur.


Bus Idle Machine Cycle : Bus Idle means neither a read or write operation occurs through the bus. The concept of Bus Idle Machine cycle is explained for DAD instruction , RST and TRAP interrupts

Fig. BUS IDLE MACHINE CYCLE for DAD instruction

DAD instruction takes 10 T-states to execute. 4 T-states for opcode fetch and 6 T-states for adding the 16-bit contents of HL register pair and given register pair. During those 6 T-states neither a memory or I/O read or write operation takes place in the bus. These T-states are referred as BUS IDLE machine cycle. In case of DAD instruction , RD and ALE signals are not activated.


Fig. BUS IDLE MACHINE CYCLE for TRAP instruction During INTERNAL OPCODE GENERATION for TRAP and RST instructions , 8085 executes BUS IDLE machine cycles. In response to TRAP interrupt , 8085 enters into BUS IDLE machine cycle and invokes RESTART instruction , stores the content of PC onto STACK , and places 0024H (Vector address of TRAP instruction) onto PC.

Interrupt Structure of 8085 Microprocessor :


TRAP (RST 4.5) :

Highest Priority , Non-maskable , Level and Edge triggered interrupt. The Level and Edge triggering means TRAP must remain HIGH till it is ACKNOWLEDGED. The Level and Edge triggering avoids FALSE TRIGGERING due to NOISE and TRANSIENTS.

Interrupt Vector Address of TRAP interrupt = 4.5 × 8 = 0024H [24H is the hexadecimal equivalent of 3610)

Two ways to reset TRAP interrupt : By applying EXTERNAL RESET signal to MICROPROCESSOR By giving a INTERNAL TRAP ACKNOWLEDGEMENT SIGNAL.

RST 7.5 :

Second highest priority , Maskable , Positive Edge Triggered interrupt

Interrupt Vector address of RST 7.5 interrupt : 7.5 × 8 = 003CH (3CH is the hexadecimal equivalent of 6010)

Interrupt enabled and disabled using SIM , EI(Enable Interrupt ) , DI (Disable Interrupt) instructions.

Two ways to reset RST 7.5 interrupt : Using SIM(Set Interrupt Mask) instruction Using INTERNALLY generated ACKNOWLEDGEMENT signal.

RST 6.5 :

Third highest priority , Maskable ,Positive Level Triggered interrupt.

Interrupt Vector address of RST 6.5 interrupt : 6.5 × 8 = 0034H (34H is the hexadecimal equivalent of 5210)

Interrupt enabled and disabled using SIM , EI(Enable Interrupt ) , DI (Disable Interrupt) instructions. RST 5.5 :

Fourth highest priority , Maskable , Positive Level Triggered Interrupt

Interrupt Vector address of RST 5.5 interrupt : 5.5 × 8 = 002CH (2CH is the hexadecimal equivalent of 4510)

Interrupt enabled and disabled using SIM , EI(Enable Interrupt ) , DI (Disable Interrupt) instructions. INTR :

Fifth(Lowest) priority , Maskable , Positive Level Triggered Interrupt

No Vector Address Available ; Vector Address to be provided by Intel 8259 (Programmable Interrupt Controller)

Interrupt enabled and disabled using SIM , EI(Enable Interrupt ) , DI (Disable Interrupt) instructions.


Fig.External Logic That gives RST 7 instruction opcode.

There are 8 software interrupts namely RST 0 to RST 7 , where 0 to 7 are called INTERRUPT NUMBERS. The vector address for the respective interrupts are found by multiplying the corresponding INTERRUPT numbers by 8. Eg. Interrupt Vector Address for RST 5 : 5 × 8 = 0028H (28H is the hexadecimal equivalent of 4010)

I/O Data Transfer Schemes : (i) Program Controlled I/O (or) Programmed I/O (or) Polling Control :

Data transfer between a microprocessor or microcontroller and I/O port only when I/O transfer instructions are executed.

The Microprocessor or Microcontroller checks the status bit of the I/O device to check its readiness before initiating data transfer. This process is called POLLING. Example : Communication between INTEL 8279 (Programmable Keyboard and Display Controller) and INTEL 8085 microprocessor or INTEL 8051 microcontroller.


(ii) Interrupt Program Controlled I/O (or) Interrupt Driven I/O :

The peripheral wanting to send a data to the microprocessor or microcontroller sends an interrupt request.

On receiving the request , the microprocessor or microcontroller completes its current execution and transfers the program control to the ISR of the corresponding requesting device.

The ISR performs the data transfer to the microprocessor or microcontroller and then returns back to its main program at the point it was interrupted. Example : Communication between INTEL 8259 (Programmable Interrupt Controller) and INTEL 8085 microprocessor or INTEL 8051 microcontroller

Hardware Controlled I/O (or) Direct Memory Access [DMA]:

The DMA Controller sends a HOLD signal to the microprocessor or Microcontroller to initiate the data transfer.

In response to the HOLD signal , the microprocessor or microcontroller sends an acknowledgement signal to DMA controller and releases its address bus , data bus and control bus which is then held by DMA controller.

The DMA controller transfers the data at high speed to the microprocessor’s or microcontroller’s RAM.

After the data transfer is completed , the DMA Controller gives back the address bus , data bus and control bus to the microprocessor or microcontroller. Example : Communication between INTEL 8237/8257 (Programmable DMA Controller) and INTEL 8085 microprocessor or INTEL 8051 microcontroller.

I/O Control by Handshake signals :

In this type , HANDSHAKE signals are used by microprocessor or microcontroller for checking the readiness of the device with which it is communicating and also synchronize the timing of data transfer.

Handshaking is the process of exchange of specific characters between sending and receiving device to get ready with necessary settings for their communication to take place. Example : Communication between INTEL 8251(Programmable USART)(or) INTEL 8255(Programmable Peripheral Interface) and INTEL 8085 microprocessor or INTEL 8051 microcontroller.

I/O Control by Ready Signal :

This type of Data transfer takes place between a slower I/O device and a microprocessor and microcontroller.

The microprocessor or microcontroller checks the readiness of the device with which it is communicating by polling its READY pin. If READY pin of microprocessor or microcontroller is activated , it means the peripheral port is ready for communication. If Ready pin is not activated the microprocessor or microcontroller generates WAIT STATES for the peripheral port to get ready for communication. Example : Communication between INTEL 8218/8219 (Programmable Bus Controller) and INTEL 8085 microprocessor or INTEL 8051 microcontroller.


Memory Organization of 8085 Microprocessor : Concept of Memory Interfacing :

8085 Microprocessor contains 16-bit address bus . So it can access upto 216 = 64 KB of memory location. But the amount of memory to be used by 8085 microprocessor depends on application.

EPROM (s) are used as PROGRAM or CODE MEMORY and RAM(s) are used as DATA MEMORY. When EPROM and RAM is used , TOTAL MEMORY SPACE IS SHARED BETWEEN THEM.

EPROM should be located from 0000H since reset address of PC is 0000H.

The main requirements for memory interfacing : (i) Select Chip (ii) Identify Register [memory location] (iii) Enable appropriate buffer.

In order to communicate with memory , it is necessary to decode the address from the microprocessor. There are two types of memory decoding techniques : (i) Absolute or Full Decoding (ii) Linear or Partial Decoding.

In Absolute or Full Decoding technique , all the HIGHER ORDER ADDRESS LINES are decoded to select the memory chip and for that purpose , appropriate logic levels must be present in the address lines. This ADDRESSING TECHNIQUE IS USED IN LARGE MEMORY BASED MICROPROCESSOR SYSTEMS.

In Linear or Partial Decoding technique , only ONE OF THE HIGHER ORDER ADDRESS LINES are used to select the memory chip and for that purpose , the address line is connected to individual CHIP SELECT pins through NOT gate. This ADDRESSING TECHNIQUE IS USED IN SMALL MEMORY BASED MICROPROCESSOR SYSTEMS. The main advantage of this technique is REDUCED COST and main disadvantage of this technique is MULTIPLE OR SHADOW ADDRESSES ARE FORMED.

Design a microprocessor system for the 8085 microprocessor such that it should contain 16 KB EPROM implemented using two 8 KB EPROM and a 4 KB RAM implemented using two 2 KB RAM using Absolute decoding technique

Memory IC A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0 Address

SA of EEPROM 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0000H


EA of EPROM 1 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1FFFH

SA of EEPROM 2

0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 2000H

EA of EPROM 2 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3FFFH

SA of RAM 1 0 1 0 X X 0 0 0 0 0 0 0 0 0 0 0 4000H

EA of RAM 1 0 1 0 X X 1 1 1 1 1 1 1 1 1 1 1 47FFH

SA of RAM 2 0 1 1 X X 0 0 0 0 0 0 0 0 0 0 0 6000H

EA of RAM 2 0 1 1 X X 1 1 1 1 1 1 1 1 1 1 1 67FFH

Design memory system for the 8085 microprocessor such that it should contain 1 KB EEPROM and 1KB RAM using Linear Decoding or Partial Decoding.

Memory IC A15 A14 A13 A12 A11 A10

A9

A8

A7

A6

A5

A4

A3

A2

A1

A0

Address

SA of EEPROM

0 X X X X X 0 0 0 0 0 0 0 0 0 0 0000H

EA of EPROM 0 X X X X X 1 1 1 1 1 1 1 1 1 1 3FFFH

SA of RAM 1 X X X X X 0 0 0 0 0 0 0 0 0 0 8000H

EA of RAM 1 X X X X X 1 1 1 1 1 1 1 1 1 1 83FFH

SA – Starting Address EA – Ending Address

Documents

Hardware Architecture of 8085 MicroprocessorInstruction with 6 T-states in Opcode Fetch Cycle : PCHL , DCX H Memory Read Machine Cycle: In this machine cycle , 8085 microprocessor