How Does an SR Latch Work?

The following page explains how SR latches work. See this page for more information about the operation of SR latches.

The following is the circuit for an SR latch, built from two NOR gates:

If we imagine the circuit starts with R and S both at zero.

• The top NOR gate connected to R is outputting a 1 at Q.
• The 1 output feeds back into the bottom NOR gate connected to S, causing it to output a zero.

If we apply a 1 to the R input (when S is zero), the top NOR gate output becomes zero.

However, this gets fed into the bottom NOR gate connected to S, which then outputs a 1.

That 1 is then fed back into the top NOR gate. Even if we now change the R input to zero, the output of the top NOR gate (Q) remains at zero because of that 1 feeding in from the bottom NOR gate.

If we set the R input to 1 again, or zero again, the Q output remains at zero. No matter what we do to R now, the output at Q is going to stick at zero.

If we apply a 1 to the S input (when the R is 0), the output of the bottom NOR gate becomes zero.

This is fed into the input of the top NOR gate. The output from the top NOR gate then becomes 1.

Even if we set the S input back to zero, the Q output at the top remains at 1. If we change S to 1 again, the Q output remains at 1. No matter what we now do the S input, the Q output will always be 1.