Sequential Ciruits

# Sequential Circuits

Sequential circuits contain memory cells as well as logic gates. The state of the memory cell is determined by previous inputs. Memory cells are implemented using flip-flops.

Flip-Flops
A flip-flop is a circuit that can store a binary value as long as power is supplied to the circuit. A flip-flop can store the binary values 1 and 0. When a value has been set, the flip-flop remains in this state (ie a 1 or 0) until it is told to change states.

A flip-flop has two ouputs, Q and Q (NOT Q), and two inputs, set (S) and reset (R). This type of flip-flop is also known as an RS flip-flop. Flip-flops are the basic buildng blcoks of comuter memory. A register is made up of a group of flip-flops to store. A 32 bit register would con-tain 32 flip-flops to store each of binary digits.

If the set input is 1 and the reset input is 0, gate 2 will have an output of 1. This can be determined by looking at the truth table for the NOR gate. If either of the NOR inputs are 1, then the output has to be regardles of the othe input. If the output from gate 2 has to be 0 and the reset value is 0, then gate 1 will produce the output of 1 (ie Q=1 and Q=0). This is called the set state (or 1-state).

If the set input changes, the output will remain the same according to the truth table rules, because the output of gate 2 will continue to be 1. As a result, the flip-flop is able to store the value that has been set. When reset value is changed, the outputs change to Q = 0 and Q = 1. This is called the clear state (or 0-state)

When implemented in the computer, both inputs remain at 0 unless the state of the flip-flop has to be changed. changing the set input to 1 casues the flip-flop to go to the set state. The set input will then return to 0. Changing the reset input to 1 will cause the flip-flop to go to the clear state. The reset input will then return to 0.

The NAND flip-flop operates with both inputs remainng at 1, unless the state needs to be changed. A 0 applied to the set input will put the flip-flop into the set state. The set input then returns to be 1. A 0 applied to the reset input puts the flip-flop into a clear state. The reset input will then return to 1.

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page revision: 6, last edited: 06 May 2010 02:35