#348 D Latch

Building a D Latch with NAND gates (74LS00).

Here’s a quick demo of it in action..

Notes

The Gated D latch:

sets the complementary outputs (Q & ~Q) based on a single input bit (“D” i.e. data)
out changes are synchronous i.e. no clocking required - they change immediately
new data is only latched when the enable input is set
if enable input is not set, the outputs (Q & ~Q) remain latched at their last value

The gated D latch is a fundamental 1-bit memory unit, and is at the core of much digital logic design.

Truth Table

E	D	Q(t+1)	~Q(t+1)
0	0	Q(t)	~Q(t)
0	1	Q(t)	~Q(t)
1	0	0	1
1	1	1	0

Logic Design

There are various combinations of gates that can be used to construct a D latch.

I’m going to test a design based on an SR NAND latch, which means a single 74LS00 (quad two-input NAND):

Alternatively this can be built based on an SR NOR latch:

Ben Eater’s Tutorials

Great explanations of the D latch and D Flip-Flop..

Construction

I’m building this first on a breadboard using a single 74LS00 (quad two-input NAND), LED indicators and manual inputs.

the enabled input uses one pole of a DPDT switch
the data input uses a push-button with the default position low with a 1kΩ pull-down resistor

NB: the pull-down resistor can’t be too stiff with the 74LS00. For example, a 10kΩ pull-down resistor allows the input to float a little too high and corrupt the input.

Breadboard

Schematic

Build