โฑ Clock Cycles and Sequential Circuit Updates
This note attempts to explain clock cycles, and how it is used in sequential circuits
๐ง What Is a Clock Cycle?
A clock cycle is the fundamental timing unit in digital systems, especially sequential circuits. It defines the rhythm at which state changes (updates) are allowed to occur.
- Clock signal: A periodic square wave alternating between high and low.
- Rising edge: Transition from low to high.
- Falling edge: Transition from high to low.
- Edge-triggered circuits: Most sequential elements (like flip-flops) update their state only on a specific edgeโusually the rising edge.
๐ Update-and-No-Update Cycle
This cycle defines when updates are allowed and when they are ignored:
| Phase | Description |
|---|---|
| Update phase | On the triggering edge, the circuit samples inputs and updates its state. |
| No-update | Between edges, the state is held constantโno updates occur. |
Any signal changes that happen between edges are ignored unless they persist until the next triggering edge. Fast transients or glitches are missed.
โก Clock Speed and Update Rate
- Clock frequency (e.g., 3 GHz) = number of cycles per second.
- Higher frequency = shorter cycle period = more updates per second.
- This enables faster processingโmore instructions or state transitions per unit time.
โ ๏ธ Stability Caveat
- Signals must settle before the next edge.
- If not, you risk setup/hold violations or metastability.
๐ง Summary
Sequential circuits synchronize state updates to clock edges. The clock cycle defines the temporal resolution of valid updates
Transients faster than the cycle period are ignored, and higher clock frequencies enable more frequent state transitionsโprovided signal stability is maintained