8085 Microprocessor

8085 Pin Description

Signals can be classified into seven groups:

1.     Power supply & Frequency signals

a)     VCC   -  +5V

b)     VSS    - Ground

c)     X1 & X2 – To provide clock signal

                                         - crystal circuit connected at these two pins       

            

• Crystal circuit provide 6 MHZ frequency signal. Internal clock generator divides oscillator frequency divided by two and provide 3MHZ frequency clock signal

d) CLK OUT - Used as system clock for other devices, the frequency is 3 MHZ

2. Control & status Signals:

             a) ALE:  It is Address Latch Enable. It goes high when operation is started by the  

                            processor. ALE is high, when address passed through address/data bus.                     

            b) RD: Read signal.  Active low signal. When it is low, it indicates that data read from                                          selected memory location or input device

c)     WR: Write signal. It is active low signal. When it is low, indicates that data  must be written to memory location or output device.

 d) IO/M  : Indicates whether I/O operation or memory operation being carried out.

 e) S0 & S1 : Status signal, which indicates type of machine cycle in progress

                      

       

f) READY – It is used by microprocessor to sense whether peripherals are ready or not for data  transfer.  It is used to synchronize slower external device to microprocessor .                     

                  - If it is low, microprocessor waits

3. Data bus & Address bus

         a) A8-A15: The upper half of the 16-bit address appears on the address lines A8-A15

         b) AD0-AD7: 8 bit data bus is multiplexed with the lower half of address bus  A0-A7

4. Interrupt signals:

          a) There are five interrupt signals: INTR, TRAP, RST 7.5, RST 6.5,   RST 5.5                                     

          b) INTA:  Interrupt acknowledge, which indicates that the processor has acknowledged  an INTR 

              interrupt.     

            

5. Serial I/O signals: SID & SOD

6. DMA signals: HOLD & HLDA

    a) HOLD – indicates that another device is requesting for the use of address bus, data bus and control bus. When microprocessor receives hold signal it  completes current  machine cycle and stops executing  next instruction. 

        b) HLDA( Hold Acknowledge)- In response to HOLD microprocessor generate  HLDA

7. Reset signals:

          a) RESET IN: lf it is low , sets the program counter to zero

          b) RESET OUT: indicates that processor is being reset