2.6 Latches¶

Toggle¶

Block diagram — Toggle

This block toggles the value of the output Q when a 0→1 flank is received on the T input.

Timing diagram — Toggle

Inputs

T — a high flank (0→1) on this input toggles the value of the output Q.

Outputs

Q — toggles its value when a high flank is received on T.

Properties: None.

SR Latch¶

Block diagram — SR Latch

This block performs a set/reset latching function between its inputs. The output Q is latched high if S is 1 (HIGH). The output is latched 0 (LOW) if R is 1 (HIGH). If both R and S are 1 (HIGH) the output is 0 (LOW). If both inputs are 0, the output keeps the last value.

Timing diagram — SR Latch

Inputs

S — sets the output Q high.
R — sets the output Q low.

Outputs

Q — 1 if S is 1, 0 if R is 1, and 0 if both R and S are 1.

Properties: None.

D Latch¶

Block diagram — D Latch

When the D input is high, the Q output follows the E input. When the D input turns off, the last value of Q is latched to Q, and the E input is ignored.

Inputs

E — the Q output follows this input when the D input is high.
D — latches the current value of E to the Q output.

Outputs

Q — the latch output.

Properties: None.