Digital Electronics
Explainer
Latch vs Flip-Flop: How Circuits Remember
Two cross-coupled NOR gates can store one bit of memory.
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Lesson notes
Key points
- Sequential circuits differ from combinational ones because their outputs depend on stored past state, not only current inputs.
- An SR latch feeds each NOR output back into the other gate, so Set stores one and Reset stores zero.
- A latch is level-triggered, changing whenever its enable is high, while a flip-flop is edge-triggered, changing only at a clock edge.
- A D flip-flop simply copies its D input to Q at each clock edge, removing the invalid SR state.
Takeaway: Latches respond to levels; flip-flops respond only to clock edges.
Quick check: When does an edge-triggered flip-flop change its output?
- At a clock edge
- Whenever enable is high
- Whenever inputs change
Sequential circuitSR latchEdge-triggeredD flip-flop
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