Hold (no change)

Also called: hold condition, hold mode, no-change condition, memory mode, store mode

The operating condition in which a latch, flip-flop or register keeps its stored value unchanged: Q⁺ = Q.

To hold is to keep the stored value exactly as it is: Q⁺ = Q. It is the condition that makes a storage element a memory at all.

How each element is told to hold:

NOR SR latch: S = R = 0.
NAND SR latch: = = 1.
Gated D latch: enable inactive (EN = 0).
JK flip-flop: J = K = 0 at the edge.
T flip-flop: T = 0 at the edge.
D flip-flop: between edges it always holds; at an edge it holds only if D happens to equal Q.
register with load enable: L = 0, so the MUX feeds Q back into D.
universal shift register: mode 00.

The key point for tracing: hold depends on the present value. The output in a hold slot equals the output just before it, so to predict Q you look back to the last set, reset, load or toggle.

Don't confuse hold with hold time. Hold time is a timing requirement: how long D must stay steady after a clock edge. Hold, here, is an input combination that means "keep Q".

And don't confuse hold with clear. Holding a 1 keeps a 1; it does not reset anything.

Worked examples

Example

Spotting the hold rows

A NOR SR latch starts at Q = 1. The (S, R) pairs (0, 0), (0, 1), (0, 0), (1, 0), (0, 0) are applied in order. Which steps are holds, and what is Q after each?

1.
(0, 0): hold. Q stays 1.
2.
(0, 1): reset. Q = 0.
3.
(0, 0): hold. Q stays 0. Same inputs as step 1, different output.
4.
(1, 0): set. Q = 1.
5.
(0, 0): hold. Q stays 1.

Example

Hold in a register

A 4-bit load-enable register holds 0101. Its data lines change to 1111 and then 0000, but L = 0 at both edges.

1.
L = 0, so each bit's MUX passes its own Q back to D.
2.
At each edge every flip-flop reloads its current value.
3.
The register still holds 0101 after both edges.

Common mistakes

Reading a hold input as "output 0". Hold keeps whatever is stored, 0 or 1.
Confusing hold with hold time.
Thinking a register holds by stopping its clock. It holds by reloading its own value.

Practice Hold (no change)

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

Latches and Flip-Flops lesson full course

And 2 more in the Latches and Flip-Flops lesson.

Registers lesson full course

Practice exams

Latches and Flip-Flops: Exam 3 · 1 question

Learn it step by step

Hold (no change) is taught in Latches and Flip-Flops, Registers and Counters.

Latches and Flip-Flops →Registers →Counters →

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