Latches and Flip-Flops

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This material was developed with funding from the National Science Foundation under Grant # DUE 1601612

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0

R

1

S

Q

Q

RESET 0 or OFF

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Click switches on and off

SET 1 or ON

1

0

Not allowed

Not legal for both latches to be turned on

RS Latch

INPUTS

OUTPUT

Latches and Flip Flops

No change

The truth table illustrates the operation of the RS Latch. The RS Latch is called a bi-state device due to the fact that it can be switched into one of two possible states SET or RESET.

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The RS Latch is constructed of cross-connected NOR gates as shown above. The Q output can start in either state ON (1) or OFF (0). If a 1 is applied to the R input the Q output will go LOW or (0) and stay in that state even if the 1 on the R input is removes. If a 1 is applied to the S input the Q output will switch to 1 even after the 1 is removed from the S input. Applying 1’s to both the R and S input is not allowed and represents an illegal input in which the output cannot be predicted.

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The RS (Rest-Set) Latch can also be controlled with LOW or 0 inputs rather than 1’s. In this case the LOW inputs will change or latch the device into the new state. Applying 0’s to both the R and S inputs is not allowed.

R

S

Inverted RS Latch

The Inverted RS Latch is constructed of cross-connected NAND gates as show below. The Q output can start in either state ON (1) or OFF (0). If a 0 is applied to the R input the Q output will go LOW or (0) and stay in that state even if the 0 on the R input is removes. If a 0 is applied to the S input the Q output will switch to 1 even after the 0 is removed from the S input. Applying 0’s to both the R and S input is not allowed and represents an illegal input in which the output cannot be predicted.

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CK

Clocked

Just like logic gates form the basic elements for combination logic circuits. Latches and flip-flops are the basic building boxes used to create storage and sequence circuits for binary information. A single latch or flip flop can store one bit of data. The main difference between the latches and flip flops is that a latch checks input continuously and changes the output whenever there is a change in input. Flip-flops use a special type of input called a clock or trigger. A change in this input is required to change or trigger the output state of the flip-flop.

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