Microprocessor Ramesh S Gaonkar

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Micro Processors and Microcontrollers



8085 INTRODUCTIONThe features of INTEL 8085 are :

It is an 8 bit processor.

It is a single chip N-MOS device with 40 pins.

It has multiplexed address and data bus.(AD0-AD7).

It works on 5 Volt dc power supply.

The maximum clock frequency is 3 MHz whileminimum frequency is 500kHz.

It provides 74 instructions with 5 different addressing

modes.



8085 INTRODUCTION

It provides 16 address lines so it can access 2^16

=64K bytes of memory.

It generates 8 bit I/O address so it can access

2^8=256 input ports.

It provides 5 hardware interrupts:TRAP, RST 5.5,

RST 6.5, RST 7.5,INTR.

It provides Acc ,one flag register ,6 general

purpose registers and two special purpose

registers(SP,PC).

It provides serial lines SID ,SOD.So serial

peripherals can be interfaced with 8085 directly.



8085 PIN DIAGRAM



Pins

Power Supply: +5 V

Frequency

Generator isconnected to

those pins

Input/Output/

Memory

Read

Write

Address latchMultiplexed

Address DataBus

Enable

AddressBus



8085 PIN DESCRIPTIONSome important pins are :

AD0-AD7: Multiplexed Address and data lines.

A8-A15: Tri-stated higher order address lines. ALE: Address latch enable is an output signal.It goes high

when operation is started by processor .

S0,S1: These are the status signals used to indicate type of

operation. RD¯: Read is active low input signal used to read data from I/O

device or memory.

WR¯:Write is an active low output signal used write data on

memory or an I/O device.



8085 PIN DESCRIPTION READY:This an output signal used to check the status of

output device.If it is low, µP will WAIT until it is high.

TRAP:It is an Edge triggered highest priority , non maskable interrupt. After TRAP, restart occurs and executionstarts from address 0024H.

RST5.5,6.5,7.5:These are maskable interrupts and havelow priority than TRAP.

INTR¯&INTA:INTR is a interrupt request signal after whichµP generates INTA or interrupt acknowledge signal.

IO/M¯:This is output pin or signal used to indicatewhether 8085 is working in I/O mode(IO/M¯=1) orMemory mode(IO/M¯=0 ).



8085 ARCHITECTURE



Arithmetic and Logical

groupAccumulator: It is 8 bit general purpose register.

It is connected to ALU.

So most of the operations are done in Acc.Temporary register: It is not available for user

All the arithmetic and logical operations are done in the

temporary register but user cant access it.

Flag: It is a group of 5 flip flops used to know status of variousoperations done.

The Flag Register along with Accumulator is called PSW

or Program Status Word.



Flag Register

- 8 bit register - shows the status of the microprocessor before/after an

operation

- S (sign flag), Z (zero flag), AC (auxillary carry flag), P (parity flag) &

CY (carry flag)

D7 D6 D5 D4 D3 D2 D1 D0

S Z X AC X P X CY

- Sign Flag

Used for indicating the sign of the data in the accumulator The sign flag is set if negative (1 - negative)

The sign flag is reset if positive (0 -positive)



Zero Flag

- Is set if result obtained after an operation is 0- Is set following an increment or decrement operation of that register

10110011

Carry Flag

+ 01001101

---------------

100000000

- Is set if there is a carry or borrow from arithmetic operation

1011 0101 1011 0101

+ 0110 1100 - 1100 1100

--------------- ---------------

Carry 1 0010 0001 Borrow 1 1110 1001



Auxillary Carry Flag

- Is set if there is a carry out of bit 3

Parity Flag

- Is set if parity is even

- Is cleared if parity is odd



Register Group

Temporary registers (W,Z):These are not

available for user. These are loaded only when

there is an operation being performed.

General purpose:There are six general purpose

registers in 8085 namely B,C,D,E,H,L.These are

used for various data manipulations.

Special purpose :There are two special purposeregisters in 8085:

1. SP :Stack Pointer.

2. PC:Program Counter.



Registers

- General Purpose Registers B, C, D, E, H & L (8 bit registers)

Can be used singly

Or can be used as 16 bit register pairs- BC, DE, HL

H & L can be used as a data pointer (holds memoryaddress)

Accumulator Flags

- Special Purpose Registers

Accumulator (8 bit register)

- Store 8 bit data- Store the result of an operation

- Store 8 bit data during I/O transfer

B C

D E

H L

Program Counter

Stack Pointer

Address 16 8 Data



Register GroupStack Pointer: This is a temporary storage memory 16 bit register.

Since there are only 6 general purpose registers, there is a need

to reuse them .

Whenever stack is to be used previous values are PUSHED on

stack and then after the program is over these values are

POPED back.

Program Counter: It is 16 bit register used to point the location

from which the next instruction is to be fetched.

When a single byte instruction is executed PC is automatically

incremented by 1.

Upon reset PC contents are set to 0000H and next instruction is

fetched onwards.



INSTRUCTION

REGISTER,DECODER &CONTROL

Instruction register:When an instruction is fetched , it is

executed in instruction register.This register takes the

Opcode value only.

Instruction decoder: It decodes the instruction from

instruction register and then to control block.

Timing and control:This is the control section of µP.Itaccepts clock input .



INTERRUPT CONTROL

It accepts different interrupts like TRAP

INT5.5,6.5,7.5and INTR.

SERIAL IO CONTROLGROUP

It is used to accept the serial 1 bit data byusing SID and SOD signals and it can be

performed by using SIM & RIM

instructions.



The Address and Data Busses The address bus has 8 signal lines A8 - A15which are unidirectional.

The other 8 address bits are multiplexed (timeshared) with the 8 data bits.

- So, the bits AD0 - AD7 are bi-directional and serve asA0 - A7 and D0 - D7 at the same time.

During the execution of the instruction, these lines carry the

address bits during the early part, then during the late parts of the execution, they carry the 8 data bits.

- In order to separate the address from the data, we canuse a latch to save the value before the function of the

bits changes.



Demultiplexing AD7-AD0

- From the above description, it becomes obvious

that the AD7- AD0 lines are serving a dual purposeand that they need to be demultiplexed to get all theinformation.

- The high order bits of the address remain on the

bus for three clock periods. However, the low order bits remain for only one clock period and theywould be lost if they are not saved externally. Also,notice that the low order bits of the addressdisappear when they are needed most.

- To make sure we have the entire address for thefull three clock cycles, we will use an external latchto save the value of AD7- AD0 when it is carryingthe address bits. We use the ALE signal to enable

this latch.



Demultiplexing AD7-AD0

8085 A15- A8

ALE

AD7- AD0Latch

A 7 - A 0

D7- D0

- Given that ALE operates as a pulse during T1, we will

be able to latch the address. Then when ALE goes low,the address is saved and the AD7- AD0 lines can be

used for their purpose as the bi-directional data lines.



Demultiplexing the Bus AD7 - AD0

The high order address is placed on the address bus and hold for 3 clk

periods,

The low order address is lost after the first clk period, this address

needs to be hold however we need to use latch The address AD7 - AD0 is connected as inputs to the latch 74LS373.

The ALE signal is connected to the enable (G) pin of the latch and the

OC - Output control - of the latch is grounded



The Overall Picture

Putting all of the concepts together, we get:

8085

A15-A8

ALE

Latch AD7- AD0 A 7 - A 0

Chip SelectionA15- A10Circuit

CS

A9- A0 1K ByteMemory

Chip

WR RD IO/M D7- D0

RD

WR

Microprocessor Ramesh S Gaonkar

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