D latch

Latches are designed to be transparent. That is, input signal changes cause immediate changes in output. Flip-flops, on the other hand, have their content change only either at the rising or falling edge of the clock signal. After the rising or falling edge of the clock, the flip-flop content remains constant even if the input changes

The D latch as shown below has an enable input. When the E input is 1, the Q output follows the D input. In this situation, the latch is said to be "open" and the path from the input D to the output Q is "transparent". Thus the circuit is also known as a transparent latch. When E is 0, the latch is disabled or "closed", and the Q output retains its last value independent of the D input.

The truth table and diagram


  • The green switch is a on/off switch (similar to a room light switch).
  • Q0 is the previous state of Q and Q0 is the previous state of Q.
  • Output Q depends on E (Enable)
    • E (Enable) high: Input passes to output
    • E (Enable) low: Latch holds its output
  • The D Latch is level sensitive and transparent.
  • Watch the video to learn how to edit the input (thick) waveforms.