Cross-Coupled Feedback

Cross-coupled gates create the feedback loop that lets a circuit remember a bit, the principle behind every latch and static memory cell.

The mechanism

Take two inverting gates and connect the output of each to the input of the other. This cross-coupling forms a loop with two stable states: output high or output low. The loop reinforces whichever state it is in, so the circuit holds a value with no clock and no external drive.

Bistability

A cross-coupled pair is a bistable element. Each stable state is self-consistent: the outputs feed inputs that regenerate the same outputs. A third, balanced point exists in theory but is unstable, so any disturbance drives the circuit to one of the two stable states.

Writing a value

To change the stored bit, an external signal must briefly overpower the feedback and force one side to the desired level. Once released, the loop latches the new value. This is how the set and reset inputs of an SR latch work.

Metastability

If a write attempt leaves the loop near its unstable balance point, it can hover there for an unpredictable time before settling. This metastable behavior underlies the setup and hold timing rules of flip-flops.

Where it appears

Cross-coupled feedback is the heart of the SR and D latches, all flip-flops, and the six-transistor SRAM cell. Wherever a static circuit remembers a bit, this loop is present.