CSCI 232 Lab #7

Build Your Own Controller

Date Due: 14 March 2012

This week we are going to build some control circuits for our hypothetical computer we are designing. (What, you say? You didn’t know we were designing a computer? Well, look at the past couple of labs…first we did ALU things, and then we did memory things. This week we are going to do CPU control kinds of things.)

One thing processors need is to know what state they are in, since that controls what operations are currently occurring. You are to build a shift register using 8 flip flops that contain a single 1 and the rest zeroes. As your clock drives the flip flops, you are to shift the 1 from one flip flop to the next, and when it shifts off the right end, you are to put it back in on the left. The output of each flip flop should connect to one of the LED’s so you should see a single green LED apparently work its way across the display until it reaches the right end, at which point it should reappear on the left. For extra credit, add additional circuitry such that when an input switch is high, the shift register resets to the initial state (1 in the left flip flop) on the next clock.

A second useful circuit is a frequency divider, since we often need to generate lower clock frequencies from higher ones. This is easy to do if you want to simply divide by a power of 2, but what if we need a different subharmonic? You are to design and implement a circuit that uses three switches to set the divider from 2 through 7 and divides the input clock by the corresponding amount. You should hook the clock to one LED, and the output to another, so that it will be easy (for low clock frequencies) to determine what the current divisor is.