Interface 8085 microprocessor

 Interfacing 8085 means the way to properly connect the microprocessor with the outside unit i.e. I/O unit and the inside unit i.e. Memory unit. So, there are two types of interfacing: I/O interfacing and Memory interfacing.

I/O interfacing means how to connect MP to the peripheral devices e.g. 8255A, 8251A etc.

Memory interfacing means how to connect MP to the memory devices e.g. RAM, ROM, EPROM etc.

For the interfacing, the remaining address pins must be decoded using the technique called "Address Decoding". This technique uses following decoders to decode:

i) Logic Gate Decoder (Simple NAND Gate decoder)

ii) Line Decoder

      - The 1-to-2 line decoder

      - The dual 2-to-4 line decoder

      - The 3-to-8 line decoder

8085 microprocessor has 16-bit address i.e. it uses 16 pins for address bus. Suppose, we have to interface with 4 KB RAM. This RAM has 12-bit address bus (2^12=4*1024 bytes). Lower 12-bits (A0-A11) of MP gets directly coonected to address pins of RAM. But, what about other upper 4-bits  (A12-A15) address of MP. These pins are the things needed to be decoded by using above decoders. 

Fig; memory interfacing using NAND gate decoder

Fig; memory interfacing using 3*8 decoder

We know that a microprocessor is the CPU of a computer. A microprocessor can perform some operation on a data and give the output. But to perform the operation we need an input to enter the data and an output to display the results of the operation. So we are using a keyboard and monitor as Input and output along with the processor. Microprocessors engineering involves a lot of other concepts and we also interface memory elements like ROM, EPROM to access the memory.

Interfacing Types

There are two types of interfacing in context of the 8085 processor.

• Memory Interfacing.
• I/O Interfacing.

Memory Interfacing:

While executing an instruction, there is a necessity for the microprocessor to access memory frequently for reading various instruction codes and data stored in the memory. The interfacing circuit aids in accessing the memory.

Memory requires some signals to read from and write to registers. Similarly the microprocessor transmits some signals for reading or writing a data.

But what is the purpose of interfacing circuit here?

The interfacing process involves matching the memory requirements with the microprocessor signals. The interfacing circuit therefore should be designed in such a way that it matches the memory signal requirements with the signals of the microprocessor. For example for carrying out a READ process, the microprocessor should initiate a read signal which the memory requires to read a data. In simple words, the primary function of a memory interfacing circuit is to aid the microprocessor in reading and writing a data to the given register of a memory chip.

I/O Interfacing:

We know that keyboard and Displays are used as communication channel with outside world. So it is necessary that we interface keyboard and displays with the microprocessor. This is called I/O interfacing. In this type of interfacing we use latches and buffers for interfacing the keyboards and displays with the microprocessor.

But the main disadvantage with this interfacing is that the microprocessor can perform only one function. It functions as an input device if it is connected to buffer and as an output device if it is connected to latch. Thus, the capability is very limited in this type of interfacing.