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Count enable

Also called: counter enable, count-enable input, enable input (counter)

A control input that lets a counter pause: with enable = 1 it counts at each clock edge, and with enable = 0 it holds its present value.

A count enable input E decides, at each clock edge, whether the counter moves:

  • E = 1: count, as usual.
  • E = 0: hold the present value.

In a synchronous counter built from T flip-flops, you AND E into every T input: T0 = E, T1 = , T2 = , and so on. With E = 0, every T is 0, so every flip-flop holds. With D flip-flops, Dᵢ = Qᵢ ⊕ (E · lower bits all 1).

The clock keeps running the whole time. Enable doesn't stop the clock; it makes every flip-flop choose "hold" at that edge. That matters: gating the clock itself with an AND gate can create short glitchy clock pulses and clock skew, which is why designers use an enable instead. It's the same idea as a register's load enable.

Count enable is also what makes cascading work: the terminal count of one counter drives the enable of the next.

CLKEQ0Q1Q2

Worked examples

Example

Reading the waveform

In the diagram, a 3-bit counter starts at 000. E at the eight edges (slots 2, 4, … , 16) is 1, 1, 0, 0, 1, 1, 0, 1.

  1. 1.

    Edges 1 and 2 (E = 1): 0 → 1 → 2.

  2. 2.

    Edges 3 and 4 (E = 0): hold at 2. The clock still ticks.

  3. 3.

    Edges 5 and 6 (E = 1): 2 → 3 → 4.

  4. 4.

    Edge 7 (E = 0): hold at 4. Edge 8 (E = 1): 4 → 5, so Q2Q1Q0 ends at 101.

Example

Enable near the wrap

A 4-bit counter with enable starts at 1110. E at the next four edges is 1, 0, 1, 1.

  1. 1.

    E = 1: 1110 → 1111.

  2. 2.

    E = 0: hold at 1111.

  3. 3.

    E = 1: 1111 → 0000 (wrap).

  4. 4.

    E = 1: 0000 → 0001.

Common mistakes

  • Thinking E = 0 clears the counter. It holds the current value; clearing is a separate input.

  • Building enable by ANDing E with the clock. That gates the clock and can cause glitches and skew; AND it into the T or D logic instead.

  • Reading E in the middle of a cycle. Only its value at the active edge counts.

Practice Count enable

Interactive questions with instant feedback and a worked solution for every wrong answer.

Combinational Logic lesson full course

Latches and Flip-Flops lesson full course

Counters lesson full course

Practice exams

Related terms

Learn it step by step

Count enable is taught in Counters.

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