D flip-flop
A D flip-flop copies its D input to Q only at the instant the clock rises, and ignores D the rest of the time. Clocked storage like this is how registers and every synchronous circuit keep step.
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How it works
On each rising clock edge: Q ← D. Between edges, D can change freely and Q does not care.
| CLK | D | Q next |
|---|---|---|
| ↑ | 0 | 0 |
| ↑ | 1 | 1 |
| 0, 1 or ↓ | x | Q (hold) |
Rising edges so far: 0.
Try it on the bench
- Set D to 1 (switch 1). Q does not change until you click the clock’s red PULSE button.
- Flip D back and forth without pulsing: Q ignores it.
- Click RUN on the clock module to clock it continuously, then flip D and watch Q follow on the next tick.
Parts on the bench
- Bench power supply
- DIP switch module
- Clock / pulse generator
- 74xx74
- 2 × Logic indicator LED
In the course
Zen’s Electronics Design Mastery, module “Digital Electronics”.