Intel 8085 microprocessor
The Intel 8085 microprocessor is an 8-bit microprocessor that contains 40 pins and operates on a power supply of +5V. These pins can be categorized into six groups.
- Address Bus
- Data Bus
- Control Signals
- Power Supply
- I/O ports
- External Signals
Explanation of Pins :
1. Pin number 01,02 (X1,X2) :-
8085 microprocessor works on the frequency of 3MHz but the minimum supply possible from outside is 5MHz so to overcome this situation two circuits are used either RC or LC circuit to break down the frequency. Hence X1 and X2 break the 6MHz frequency in half and make it usable to the system.
2. Pin number 03(Reset OUT):-
This signal shows that the microprocessor is reset now and is used to reset other devices also which are connected to the microprocessor.
3. Pin number 04,05 (SOD,SID) :-
The 8085 has 2 signals to implement the serial input-output transmission – (i.) SID(Serial Input Data) and (ii.) SOD(Serial Output Data). In serial transmission, data bits are sent over a single line i.e. only one bit is used at a time either for input or output such as the transmission over the telephone lines.
4. Pin number 07,08,09(RST 7.5, RST 6.5, RST 5.5) :-
RST(Restart), these pins are used as restart interrupts. These are also known as vectored interrupts because they transfer the program control to specific locations. In priority order, they are above the INTR(interrupt) signals.
5. Interrupt :
Intel 8085 microprocessor has 5 interrupt signals that can be used to interrupt a program execution. These are external signals. The various interrupts are as follows :
- (a.) Pin Number 06 (TRAP):
- This is an active-high input signal and a non-maskable interrupt with the highest priority. When the TRAP pin goes high, the microprocessor goes to an interruption state. Call location of TRAP interrupt is 0024H.
- (b.) Pin Number 10 (INTR) Interrupt request :
- This is used by the programmer as a general purpose interrupt .
- (c.) Pin number 11 (INTA):
- INTA(Interrupt Acknowledge) is an output signal. It is generated by microprocessor to acknowledge and interrupt.
6. Pin Number 12 to 19 (AD0-AD7) (Multiplexed Address/Data Bus) and Pin Number 21-28 (A8-A15) (Address Bus) :-
8085 microprocessor has 16 pins used as signal lines that are used as address bus; These 16 lines are split into two equal segments :
- (a.) A8-A15: These are unidirectional pins and used for the MSB(Most Significant Bits) which is the higher-order address of a 16-bit address.
- (b.) AD0-AD7: These are dual-purpose signal lines i.e. they are used for both the data bus as well as the lower order address bus. However, the lower-order address bus can be separated from these signals by using a latch.
7. Pin number 20(Vss) :
It is the ground reference for 8085 microprocessor.
8. Pin number 31(WR
– Write) :
This is an active low write control signal. When the data on the data bus is to be written into a selected memory the WR signal is in active condition.
9. Pin number 32(RD – Read) :
This is an active low read control signal. When the data is available in the data bus or the selected I/O or memory devices are reading the data the RD is in active condition.
10. Pin number 36(RESET IN) :
When the signal in on RESET PIN goes low, the PC register (Program Counter) is set to zero, the buses are in tristate and the microprocessor unit is reset.
11. Pin number 37( CLK OUT : Clock Output ) :
CLK OUT is used as the system clock for another connected devices.
12. Pin number 38 (HLDA : Hold Acknowledge) :
It responds to the HOLD request. This pin is used in data transfer through direct memory access. When the microprocessor receives a HOLD signal from the external device this active high output signal will send a request to the device and receives the request and gives control of buses to the external device.
13. Pin Number 39 (HOLD) :
When the HOLD pin is activated by an external device or signal generated by HLDA, the microprocessor withdraws the control of address and data buses and gives access to the external device.
14. Pin number 40 (Vcc):
It gives power supply of +5V to the microprocessor.
15. Pin number 30 (ALE) :
When the ALE signal shows high, this indicates that the address bits(A8-A15) are in use and when the ALE signal shows low, this indicates that the data bits (AD0-AD7) are in use.
16. Pin number 35 (Ready) :
This pin is used to synchronize slower peripherals of the microprocessor and it actually delays the microprocessor’s RD or WR signals until a slow responding peripheral is ready to accept the data. The Microprocessor enters into the wait state when READY signal is low.
17. Pin numbers 33, 29 (S1 and S0) :
These are known as the status signals in 8085 microprocessor. They can identify various operations but are rarely used.
18. Pin number 34 (IO/M) :
This status signal differentiates between the memory operations and the input/output data. It combined with RD & WR to generate input/output and memory control signals.
These are the 40 pins used in the 8085 microprocessor to perform various functions. Read them again and again in a given order, you will never forget this topic.