Microprocessor Architecture

Intel 8085 Pin Configuration



The 8085  Buses

n The 8085 is an 8-bit general purpose microprocessor that can address 64K Byte of memory.

n It has 40 pins and uses +5V for power. It can run at a maximum frequency of 3 MHz.

nThe pins on the chip can be grouped into 6 groups:

n Address Bus.

n Data Bus.

n Control and Status Signals.

n Power supply and frequency.

n Externally Initiated Signals.

n Serial I/O ports.

The Address and Data Bus Systems

n The address bus has 8 signal lines A8 – A15 which are unidirectional.

nThe other 8 address bits are multiplexed (time shared) with the 8 data bits.

n So, the bits AD0 – AD7 are bi-directional and serve as A0 – A7 and D0 – D7 at the same time.

n 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.

n In order to separate the address from the data, we can use a latch to save the value before the function of the bits changes.

The Control and Status Signals

n There are 4 main control and status signals. 

These are:

nALE: 

                Address Latch Enable. This signal is a pulse that become 1 when the AD0 – AD7  lines have an address on them. 

                It becomes 0 after that. This signal can be used to enable a latch to save the address bits from the AD lines.

nRD: Read. Active low.

nWR: Write. Active low.

nIO/M: 

            This signal specifies whether the operation is a memory operation (IO/M=0) or an I/O operation (IO/M=1).

nS1  and S0 : Status signals to specify the kind of operation being performed. Usually not used in small systems.