Computer Science 324
Computer Architecture

Mount Holyoke College
Fall 2007

Lab 4: Sequential Circuits
Due: 4:00 PM, Thursday, October 18, 2007

In Lab: More Complex Circuits

Construct circuits using TTL logic (the chips, wires, breadboards, etc.) for the following. First, draw chip-numbered pin diagrams of your circuits. Once you have done this (and, ideally, gotten a stamp of approval), you may construct and test your circuits. Please demonstrate your circuits no later than 4:00 PM, Thursday, October 18. You should work in groups of two on the in-lab portion of this assignment.

  1. Construct a 3-bit adder/subtractor along the lines of the one we discussed in class. Use three switches for each of your 3-bit inputs, and an additional switch to control the line that selects addition or subtraction. Use 4 of the LEDs to display your 3-bit answer and the carry out bit.
  2. We looked at the S-R Latch built from NOR gates in class. Construct an S-R Latch using only NAND gates. Use two switches for the S and R inputs, and display Q and Qbar using LEDs.
Homework Problems

Please complete the following problems on paper. You may verify your work using TTL logic or LogicWorks. Please work individually on this part.

  1. As you saw in the lecture assignment from Lecture 10, it is sometimes necessary to add 1 to a number, for example, a program counter. You constructed, for the lecture assignment, a device that is less expensive than a ripple-carry adder, that adds one to a value that is provided (D0..D3) and provides the result on the output (Q0..Q3). Now, construct a similar device for adding 1 to a gray-code value. (Hint: you may find the circuit from the original to be a useful building block.)
  2. In class we learned about clocked D-type flip-flops. Such a device has inputs D and CLK, and output Q. Occasionally, it is useful to explicitly set (S) or reset (R) Q without regard to D. For example, it might be useful to clear (reset) all of memory without explicitly providing data to each bit.
  3. Suppose you have several T-type clocked flip-flops with Q and Qbar and asynchronous set and reset.
  4. Using any of the flip-flops, latches, and counters that we have discussed in class as building blocks, construct a circuit that takes input from a single switch and produces a single output. After the first, second, and third times that the switch goes from 0 to 1, the output is 0. The fourth time that the switch goes from 0 to 1, the output becomes 1. The fifth, sixth, and seventh times the switch goes from 0 to 1, the output should be 0. The eighth time, the output is 1, and so on. In other words, the output is 1 exactly when the switch has been moved from 0 to 1 a number of times that is divisible by 4.