CS 14 -- Lab 2

Today, you will design and begin to implement a 4-bit binary counter. This is a two week project. It is likely that you'll spend most of today's lab time solving the problem of how this sequential logic circuit will work, and you will not finish building that circuit until next week. Be sure to read the whole lab before you start working!

The problem to be solved

Your goal is to build a 4-bit binary counter. More specifically, you want a circuit that is built around 4 1-bit memory elements, where each element holds one of the bits of the counter. The 4 bits (that is, the contents of the 4 memory elements) should be connected to LEDs so that you can see the current number in the counter.

This counter should depend on a clock. When the clock is cycled (made to be high and then low again), the counter should increment by one. When the counter reaches the binary value 1111, at the next clock cycle, it should "wrap around" to 0000.

When the clock is cycled, the new values that are to be stored in the memory elements must be determined by the old values that are coming out of the memory elements. That is, you must build circuitry that determines what the value of each bit will next be, when the clock is cycled, based on the current values of the bits.

New devices

There are new devices that you will need in order to complete this lab:

• The 74LS273 (a.k.a. "273") octal D Flip-flop chip: This chip, for which a data sheet is attached, contains 8 flip-flops. There is one, shared clock for all flip-flops on a given chip; There is also one, shared clear; when activated (by setting it to be low), this line will force all of the flip-flops in that chip to store 0.
  • IMPORTANT NOTE ON CLEAR LINES: If the clear line is left open (i.e. not explicitly connected to power or ground), then the chip will behave unpredictably! That means that you can't simply connect the clear line directly to high or low, or else you might clear all of the flip-flops accidentally. Instead, connect the clear line to a pair of NOT gates, which will ensure that some valid input is always being provided to that clear line.
  • IMPORTANT NOTE ON UNUSED FLIP-FLOPS: If the D line to a flip flop is left open, the chip may behave unpredictably. Be sure to ground any unused D lines on the chip to avoid problems.
• Pushbuttons: These little devices allow you install, on your board, a button that will close a part of your circuit when pressed. When the button is not being pressed, the circuit will be open (that is, unconnected). Since our simple gates behave as though an open line is one that it powered, you must consider design your circuit as though pressing the button sets the line to low, and otherwise the line is high.

Some tips on building

Here are some useful ideas that will be help in designing and implementing your counter. Be sure to read these carefully!

• Creating a clock: You should begin by making a clock circuit, which you will use to drive the clock line on the 273 chip. The clock will itself be a memory element --- specifically, an SR latch. You will want to use pushbuttons to control both the S and R lines of this latch, thus allowing you to alternate between buttons when you want to cycle the clock. Note that pressing the buttons will make the Q output alternately high and low --- exactly what we need in a clock line. This kind of circuit is known as a debounced switch. (No, I do not know the etymology of that term.)

  Note that you should invert the pushbuttons with NOT gates before connecting them to the latch input lines. Recall that when a pushbutton is not being pressed, that line is open, and will thus behave as though it is high (i.e. powered). However, the latch is closed when its inputs are low. Thus, we need to invert those pushbuttons in order to have them control the latch correctly, opening it when a button is pressed.

• Incrementally building up the counter: Don't try to design or wire all 4 bits of your counter at once! That would be like writing a large program without compiling and debugging the small parts...and we wouldn't want to do that now, would we?

  Begin by building a 1-bit counter; yes, one that simply alternates between 0 and 1. Figure out the input to this single memory element based on its own current output.

  Graduate to a 2-bit counter. Here, you'll have two inputs, D0 and D1, and their two corresponding outputs, Q0 and Q1. Figure out what the input for D1 should be based on the current Q0 and Q1 values.

  Design and test these smaller circuits! You may find that something is wrong with your design, and it will be easier to fix with the smaller cases. Once you have ensured that you've got the right idea, you can work your way up to a full, 4-bit counter.

Finishing up

The usual rules for finishing your work apply:

• When done working in the lab, clean up. Store your board (be sure that your name is on it), put away wires, probes, excess chips, alligator clips, wire trimmers, chip pullers, and any other devices that you've used. If you have created garbage by cutting and stripping wire, throw it out.
• Show your work to the professor or the TA. You'll need to demonstrate that your counter proceeds through all 16 possible values correctly, and wraps around when it should.