Lab 4 - Latches and Flip-Flops

In this lab, you will build a working D-type flip-flop.

Step 1: Build an S–R latch

Build an S–R latch on yor protoboard using the following circuit diagram:

Image from Wikipedia

Test your S–R latch. How do you know it is storing a value?

Step 2: Build a D latch

Add the necessary gates to turn your S–R latch into a D latch. Recall that a D latch has two inputs and two outputs:

input $D$

The data value that should be stored in the latch

input $C$

The clock input. When this pin is high, the value on $D$ is stored in the latch. If $C$ is low, then the stored value should not change. Note that this input is sometimes called $E$, for “enable.”

output $Q$

The stored value in the latch

output $\bar{Q}$

The inverse of the stored value

Test your D latch to verify that it stores the correct value, and only updates the stored value when the clock input is high.

Step 3: Drive your D latch with a clock

Connect the $C$ input of your D latch to the clock signal on the left side of your protoboard. You should hook a logic indicator light up to the clock input so you can monitor it. Set your clock to a low frequency (Hz, not KHz, and 1, not 10 or 100).

With the clock set slow enough, you should be able to watch the value on $D$ update the latch state only when the clock is high.

What happens when you change $D$ while $C$ is still high?

Step 4: Build a D flip-flop

Build a second D latch and connect it to your first D latch to build an edge-triggered flip-flop. The following figure shows a D flip-flop that stores an input value on the clock’s falling edge. How would you build a flip-flop that stores on the rising edge instead?

Image from Patterson & Hennessy

Citations

Acknowledgment: This lab is adapted from a lab introduced to Grinnell College by Marge Coahran and updated by Janet Davis.

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