DIGITAL LOGIC - MEMORY DEVICES

1.13 DIGITAL LOGIC - MEMORY DEVICES

Memory devices can be constructed using the digital logic devices pre­viously discussed. These devices can be used to store (remember) the 1's and 0's value for later use.

 Set - Reset Type Flip-Flop

A very simple and common memory device is known as a flip-flop. It can be constructed many ways. Shown below is a flip-flop constructed with two NOR gates. The input and 

outputs are as follows:

 Set     - S - The input of a flip-flop. The value of the input  can be a 1 or a 0. Before the value is input, the flip-flop must first be reset.

Reset      - R      - A 1 must be input to the flip-flop to reset (or clear) it to 0 before 

the set can be used to input the value (1 or 0) to be stored.

Output -    Q    -  "Q" is the common terminology for output of a flip-flop.

                Q     The bar over the Q indicates the opposite of Q (i.e. if Q is a 1, Q is a 0 and vice versa). 'q is pronounced "Q not".

The symbol and the timing diagram for a flip-flop is shown below.

Figure : Digital logic – memory devices