NAND Gate Bistable Pull Up Resistors

This type of flip-flop bistable is also known as an SR bistable, which has two inputs known as SET and RESET. There is usually a line above them to signify that they are active low. This means that we have to use a pull-up resistor to establish them at logic 1 state, and a momentary connection to ground activates their function. We usually use a non-locking switch for making the momentary connection.

Bistable Logic

If SET is pressed and RESET is open, then Q=1, and /Q=0

If RESET is pressed and SET is open, then Q=0, and /Q=1

If SET and RESET switches remain open, then the value of the outputs depends upon what was previously set, and there is no change.

If both SET and RESET switches are simultaneously closed, then it can cause instability. Therefore, this is an undesirable condition to avoid.

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