8085 PIN ARCHITECTURE

8085 PIN ARCHITECTURE

the pin diagram of 8085 Microprocessor − 

The pins of a 8085 microprocessor can be classified into seven groups − 

Address bus 

A15-A8, it carries the most significant 8-bits of memory/IO address. 

Data bus 

AD7-AD0, it carries the least significant 8-bit address and data bus. 

Control and status signals 

These signals are used to identify the nature of operation. There are 3 control signal and 3 status signals. 

Three control signals are RD, WR & ALE. 

• RD − This signal indicates that the selected IO or memory device is to be read and is ready 

for accepting data available on the data bus. 

• WR − This signal indicates that the data on the data bus is to be written into a selected memory 

or IO location. 

• ALE − It is a positive going pulse generated when a new operation is started by the microprocessor. When the pulse goes high, it indicates address. When the pulse goes down it indicates data. 

Three status signals are IO/M, S0 & S1. 

IO/M 

This signal is used to differentiate between IO and Memory operations, i.e. when it is high indicates IO operation and when it is low then it indicates memory operation. 

S1 & S0 

These signals are used to identify the type of current operation. 

Power supply 

There are 2 power supply signals − VCC & VSS. VCC indicates +5v power supply and VSS indicates ground signal. 

Clock signals 

There are 3 clock signals, i.e. X1, X2, CLK OUT. 

• X1, X2 − A crystal (RC, LC N/W) is connected at these two pins and is used to set frequency 

of the internal clock generator. This frequency is internally divided by 2. 

• CLK OUT − This signal is used as the system clock for devices connected with the 

microprocessor.

Interrupts & externally initiated signals 

Interrupts are the signals generated by external devices to request the microprocessor to perform a task. There are 5 interrupt signals, i.e. TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR. We will discuss interrupts in detail in interrupts section. 

• INTA − It is an interrupt acknowledgment signal. 

• RESET IN − This signal is used to reset the microprocessor by setting the program counter 

to zero. 

• RESET OUT − This signal is used to reset all the connected devices when the microprocessor 

is reset. 

• READY − This signal indicates that the device is ready to send or receive data. If READY is 

low, then the CPU has to wait for READY to go high. 

• HOLD − This signal indicates that another master is requesting the use of the address and 

data buses. 

• HLDA (HOLD Acknowledge) − It indicates that the CPU has received the HOLD request and it will relinquish the bus in the next clock cycle. HLDA is set to low after the HOLD signal is removed. Serial I/O signals 

There are 2 serial signals, i.e. SID and SOD and these signals are used for serial communication. 

• SOD (Serial output data line) − The output SOD is set/reset as specified by the SIM 

instruction. 

• SID (Serial input data line) − The data on this line is loaded into accumulator whenever a RIM 

instruction is executed.