SR Latch with Cross-Coupled NOR Gates

Understanding how feedback creates memory in digital circuits

SET Input (S)
0
RESET Input (R)
0
Current State
HOLD
Outputs maintain previous values
Q
0
Q_N
1
Cross-Coupled NOR Gate Structure
R 0 1 Q_N NOR Q 0 S 0 0 Q NOR Q_N 1 X Q → NOR2 Q_N → NOR1 Q = NOR(R, Q_N) Q_N = NOR(S, Q) MEMORY Feedback creates memory: Q feeds to NOR2 Q_N feeds to NOR1

Truth Table

S R Q Q_N State
0 0 Q Q_N HOLD (Memory)
1 0 1 0 SET
0 1 0 1 RESET
1 1 0 0 INVALID
Why is this "Memory"?
When S=0 and R=0, the outputs depend on the feedback connections. Q feeds back to the bottom gate, and Q_N feeds back to the top gate. This X-shaped cross-coupling creates a stable loop that remembers the last state.

How It Works

Current: HOLD Mode
With S=0, R=0: Both NOR gates look at their feedback inputs. Since Q=0 feeds into the bottom NOR and Q_N=1 feeds into the top NOR, the outputs remain stable. The circuit "remembers" its current state!
Key Insight: Cross-Coupling
The X-shaped cross-coupled feedback is what creates memory. Q feeds to NOR2's input, and Q_N feeds to NOR1's input. This forms a bistable circuit that can rest in either state (Q=0 or Q=1) indefinitely.

Activity Log

--:-- Ready. Toggle S and R inputs to explore latch behavior!