Skip to content
Related Articles

Related Articles

Latches in Digital Logic
  • Difficulty Level : Medium
  • Last Updated : 12 Dec, 2019

Latches are basic storage elements that operate with signal levels (rather than signal transitions). Latches controlled by a clock transition are flip-flops. Latches are level-sensitive devices. Latches are useful for the design of the asynchronous sequential circuit.

SR (Set-Reset) Latch – SR Latch is a circuit with:
(i) 2 cross-coupled NOR gate or 2 cross-coupled NAND gate.
(ii) 2 input S for SET and R for RESET.
(iii) 2 output Q, Q’.

1 0 Set
0 1 Reset

Under normal conditions, both the input remains 0. The following is the RS Latch with NAND gates:

Case-1: S’=R’=1 (S=R=0) –
If Q = 1, Q and R’ inputs for 2nd NAND gate are both 1.
If Q = 0, Q and R’ inputs for 2nd NAND gate are 0 and 1 respectively.

Case-2: S’=0, R’=1 (S=1, R=0) –
As S’=0, the output of 1st NAND gate, Q = 1(SET state). In 2nd NAND gate, as Q and R’ inputs are 1, Q’=0.

Case-3: S’= 1, R’= 0 (S=0, R=1) –
As R’=0, the output of 2nd NAND gate, Q’ = 1. In 1st NAND gate, as Q and S’ inputs are 1, Q=0(RESET state).

Case-4: S’= R’= 0 (S=R=1) –
When S=R=1, both Q and Q’ becomes 1 which is not allowed. So, the input condition is prohibited.

The SR Latch using NOR gate is shown below:

Gated SR Latch –
A Gated SR latch is a SR latch with enable input which works when enable is 1 and retain the previous state when enable is 0.

Gated D Latch –
D latch is similar to SR latch with some modifications made. Here, the inputs are complements of each other. The design of D latch with Enable signal is given below:

The truth table for the D-Latch is shown below:

Enable D Q(n) Q(n+1) STATE
1 0 x 0 RESET
1 1 x 1 SET
0 x x Q(n) No Change

As the output is same as the input D, D latch is also called as Transparent Latch. Considering the truth table, the characteristic equation for D latch with enable input can be given as:

Q(n+1) = EN.D + EN'.Q(n)

DIGITAL ELECTRONICS – Atul P. Godse, Mrs. Deepali A. Godse

Attention reader! Don’t stop learning now. Get hold of all the important CS Theory concepts for SDE interviews with the CS Theory Course at a student-friendly price and become industry ready.

My Personal Notes arrow_drop_up
Recommended Articles
Page :