In this lab, you will build your understanding of memory elements by building an S–R latch, a D latch, and a D-type flip-flop. In the last step, you can implement either a simple register file or a “neat” light display using latches.
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?
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:
Test your D latch to verify that it stores the correct value, and only updates the stored value when the clock input is high.
Connect the 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 on the
TTL option, square wave, and 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 update the latch state only when the clock is high.
What happens when you change while is still high?
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.
Once you have completed this step, please have Charlie or a mentor sign off on your circuit.
Image from Patterson & Hennessy
For the final step of this lab, you may choose one of the two exercises. You must complete at least one, but you do not need to complete both to earn full credit on the lab.
Once you have completed one of the options, please have Charlie or a mentor sign off on your circuit.
Chaining two D latches together does something interesting with your data input. Every time the clock “ticks,” the value moves from the first D latch to the second. You can use this behavior to drive LEDs in an interesting pattern.
Extra Challenge: Can you hook the latches up in a loop so a single light moves through the latches over and over again? You will need to disconnect your latches from each other to initialize the stored state in each latch, then hook them together in a loop and start the clock without turning off the board.
Using the two D latches you built for your flip-flop, implement a simple two-bit register file. Your circuit should have the following inputs and outputs:
This circuit will require a simple decoder and multiplexor. The one bit versions of these components can be built with very few gates, so try to think up a simple implementation before building your circuit.
Acknowledgment: This lab is adapted from a lab introduced to Grinnell College by Marge Coahran and updated by Janet Davis.