Breadboard Flip-Flops & Latches

Flip-flops are bi-stable (two state) devices:

• Most basic type of 1-bit memory.
• The internal state is “remembered” until changed by an input pulse, or until the power is removed.
• Different types of flip-flops are build from combinations of logic gates.
• Also called bi-stable Latch.

SR/RS Type

SR (Set-Reset) flip-flop is one of the simplest sequential circuits:

• Two inputs Set (S) and Reset (R), and two outputs (Q, and negated Q).
• Output set in one of two states, and will remain latched indefinitely.

Schematic for an SR Flip-Flop and an RS Latch

Build from to NAND/NOR gates:

• The output of each gate is connected to an input of the other gate (feedback loop, cross-coupling)
• 1. Low-activated SR Flip-Flop (“NOT” indicated by the bar above the letter)
• 2. High-activated RS Latch

List of required components to build an RS Latch with a breadboard:

Pcs.	Description
1	74xx02 Quad 2 input NOR Gates
2	COM-00097 Switch
2	560Ω Resistor
2	LED (green)

RS Latch circuit

Setup on a breadboard

Truth table for an RS Latch:

S	R	Q	!Q	Comment
0	1	1	0	(a) Q is set to 1 by 1 on S
0	0	1	0	(b) no change after set
1	0	0	1	(c) Q is rest to 0 by 1 on R
0	0	0	1	(d) no change after reset
1	1	0	0	(e) not allowed (both outputs at zero)
0	0	?	?	(f) if both inputs change from 11 to 00, output is indeterminate

References

• Datasheet SN74LS02N, Texas Instruments
• SR Latch, Ben Eater
• Sequential Logic Circuits

D Latch

Circuit symbols for D type devices:

A D latch is like an RS latch with only one input:

• Overcomes the drawbacks of the SR/RS type devices (e,f).
• Does not respond to a data input D unless the enable input EN is 1.
• While the circuit is enabled the D input sets the outputs Q, and de-activating the D input resets the circuit.
• If the circuit is not enabled then the Q outputs stay latched, unresponsive to the state of the D input.

List of required components to build a D Latch:

Pcs.	Description
1	74xx02 Quad 2 input NOR Gates
1	74xx08 Quad 2 input AND Gates
2	COM-00097 Switch
3	560Ω Resistor
3	LED (green/red)

D Latch circuit

Setup on a breadboard

Truth table:

EN	D	Q	!Q
0	0	latch	latch
0	1	latch	latch
1	0	0	1
1	1	1	0

References…

• Datasheet SN74LS08N, Texas Instruments
• D Latch, Ben Eater
• Gated D Latch, Wikipedia

D Flip-Flop

List of additional components for the clock input CLK:

Pcs.	Description
1	1µf ceramic capacitor
1	1kΩ resistor

Setup on a breadboard, and a schematic for to modify the EN input to become CLK

References …D Flip Flop, Ben Eater