# Decoding Circuits Made by Adham Barghouti 20070316.

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Decoding Circuits Made by Adham Barghouti 20070316

What is a Decoding Circuit?????? Its a circuit designed to choose a memory chip from a dozen of memory chips. For Example if we had the following Memory chips how will the system know which memory you want to Read/Write on. M1M2M3 OR

Another way of understanding what the Decoding circuits that its the circuit connecting the Microprocessor with the memory via the Address bus thus giving each memory chip a unique address. 8088 Decoding Circuit Address Bus Memory chip CS CS or Chip Select is a one bit input that enables the memory on 0 or disables it on 1.

The Decoding circuit uses the Addresses in a way in which to select a certain memory chip by having the output of the decoding circuit equal to 0. For Example The only for this OR gate to have an output of 0 is for all inputs to be 0. Thus it can be said that the address that this small decoding circuit is 000. Memory chip CS

A decoding circuit can be any logic components. It can be a simple OR or NAND gate, Decoder or any other selecting method. The most common decoding circuits are NAND Gates and Decoders What is the Address that the following Decoding circuit is addressing????? Memory chip CS A0A1A2A3A4A5A6A7A0A1A2A3A4A5A6A7 First the output of the NAND gate has to be 0 in order to select the desired memory. And therefore A 0, A 3, A 5 should be 0. And the rest should equal to 1. So the address for this circuit is 11010110 which is D6h.

Using the Decoder is Easier Memory chip 1 CS Memory chip 2 CS Memory chip 3 CS Memory chip 4 CS 2x4 Decoder A0A0 A1A1 As you see using only A 0 and A 1 only as inputs for the decoder and the other address lines connected to the enable of the decoder we are able to Address 4 different memories A2A3A4A5A6A7A2A3A4A5A6A7 Try finding the Addresses of each memory!! Memory 1 D4h Memory 2 D5h Memory 3 D6h Memory 4 D7h