D Flip Flop Basics and Comparisons | Generated by AI
Answer: D flip-flop
Why it’s the right choice
- Function: A D (data or delay) flip-flop stores exactly one bit. On the active clock edge (rising or falling, depending on the design), it samples the D input and updates Q to that value. Between clock edges, Q holds its previous value.
- Behavioral rule: Q_next = D at the clock edge; otherwise Q stays unchanged. This “sample-and-hold” behavior is exactly what you want for registers and pipelines.
How it differs from the others
- RS flip-flop: Has separate Set and Reset inputs and an invalid condition (S=R=1). Not ideal for general data storage in synchronous designs.
- JK flip-flop: Generalizes RS; J=K=1 causes toggle. More flexible but more complex and rarely used for standard data registers.
- T flip-flop: Toggles when T=1 on the clock edge, holds when T=0. Great for divide-by-2 counters, not for arbitrary data storage.
Key details about D flip-flops
- Edge-triggered vs latch: A D flip-flop is edge-triggered (updates only on the clock edge). A D latch is level-sensitive (transparent while enable is asserted). When people say “directly transfers the input to the output when clocked,” they typically mean the D flip-flop captures D at the clock edge.
- Timing requirements: Correct operation requires meeting setup time (D stable before the edge) and hold time (D stable after the edge). Violating these can cause metastability.
- Variants: Often include asynchronous preset (set) and clear (reset) pins for initializing Q.
- Common uses: Building n-bit registers (n DFFs in parallel), pipeline stages, state registers, shift registers (often with a MUX for serial/parallel load), synchronizers (in pairs) for clock-domain crossings.
Rule-of-thumb
- If you need a storage element that “just stores the bit you present at the clock,” use a D flip-flop.