What is Hold Time?

Hold Time (Thold) is the minimum amount of time that the input data must remain stable after the active clock edge (rising edge or falling edge) so that a flip-flop can correctly capture the data.

Simple Definition

Hold Time is the minimum time after the clock edge during which the input data must not change.

If the data changes too soon after the clock edge, the flip-flop may capture incorrect data or enter a metastable state.

Timing Diagram

Clock:  ____/‾‾‾\\____/‾‾‾\\____

Data :  -----1-----------
               ↑<--Thold-->
           Clock Edge

The data must remain stable during the hold window after the clock edge.

Example

Suppose a flip-flop has:

Hold Time = 1 ns

If the clock edge occurs at:

10 ns

Then the data must remain stable until:

10 ns + 1 ns = 11 ns

Any data change before 11 ns may cause a hold violation.