8085 Microprocessor

• It is a 8 bit microprocessor
• It was invented in 1974
• It is 40 pin IC
• 64 KB RAM
• Input voltage 5 volt DC supply

Below is pin description of  all 40 pins

1. Pin 1 & 2:  X1 & X2 is a clock frequency pin , oscillator is connected on this pin to generate clock pulse . Clock frequency upto 8 Mhz .
2. Pin 3 : Reset out , by using this pin micro controller can reset other peripheral.
3. Pin 4 & 5 : SOD & SID these pins are use as serial data communication, SOD= serial output data line , SID= serial input dataline.
4. Pin 6 to 10 : These are interrupt lines of microprocessor.  

TRAP : It is the highest priority interrupt.
RST 7.5, RST 6.5, RST 5.5 - These are high to low priority after trap interrupt respectively.
INTR : This is lowest priority pin .

   5. Pin 11: INTA this pin is interrupt acknowledgement pin .
   6. Pin 12 to 19 : AD0 to AD7 these are bidirectional pin , these are used for first 8 bit address and and 8 bit data pins.
    7. Pin 20 : This pin is use for 5 volt dc supply to microprocessor.
    8. Pin 21 to  28 : A8 to A15 these are address lines of microprocessor.
    9. Pin 29 and 33 : S0 and S1  these are use as status signal ,
                 S0      S1
                 0        0            this shows no operation
                 0        1            Read  instruction
                 1        0           Write instruction
                 1        1            Opcode fetch



10. Pin 30: ALE ( adderss latch enable ) this is use for separate address and data from AD0 to AD 7
11. Pin 31 :  WR : this is write instruction given by microprocessor to memory or input / output, this depend on IO/ M pin status.
12. Pin 32: RD : this is read instruction given by microprocessor to memory or input / output, this depend on IO/ M pin status.
13. Pin  34 : IO / M  this is pin use to indicate that which of these memory or IO device is  in use for
microprocessor.
14. Pin 35, 38, 39, : READY , HLDA , HOLD
      these are use for DMA transfer .
15 Pin 36: RESET IN this pin use to reset 8085.
16. Pin 37: CLOCK OUT  this pin use by 8085 to gives clock pulse to other devices.
17. Pin 40: VCC : this is ground pin with respect to 5 volt supply pin no.20.

Architecture of 8085

Description of 8085 -

• Interrupt control unit- This unit is use to deal with all types of interrupts and also gives interrupt service acknowledgement from its pin INTA.
• Serial I/O control unit- This unit is use to deal with all serial input and output data from microprocessor.
• Registers - There are six registers  B ,C ,D, E, H ,L all are 8 bit register, we can use them alone for holding 8 bit data whereas to handle 16 bit data we use pair of them

        BC       16bit

        DE       16bit

        HL       16bit
     
    SP(stack pointer 16 bit) - This is special purpose register , it stores the address of last program request in a stack . Stack is a reserved memory to store data.

    PC(program counter 16 bit)- This is use to store address of the  instruction to be fetched  in the program.

   Increment / Decrement  adderss latch - It is not use for programmer , it only for 8085 to store different address.

• Accumulator(8 bit ) - It is special type of register, all arithmetic operation like add, subtract, are perform in it. It also known as A register.
• Flag register(8 bit) - Flag register shows status of the 8085 program is running , it shows by 8085 , fig below shows 5 flag register use in 8085.

  

S(sign bit)- It show result is positive or negative. S=0 (positive)    S= 1 (negative)

Z(zero bit)- It show result is zero or not.

AC(auxiliary carry)-That is carry from the 3rd bit to 4th is also indicated.

P(parity bit)-It indicates whether the result contains odd number of 1s or even no. of 1s.

CY(carry flag)-It indicates whether there is carry or not after an arithmetic and logical operation,

• Timing and control unit- This unit is brain of the 8085, it gives all control related signals and all that .DMA transfer and many other work done by this unit.
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