CSE370 Assignment 6

Distributed: 10 May
Due: 16 May

Reading:

Chapter 7 (pp. 337-345) and Chapter 8 (pp. 383-395, 400-412) of Katz.

Exercises:

1. Problem 6.22 of Katz.
2. A 4 bit Linear Feedback Shift Register (LFSR) is a counter which cycles through 15 of the possible 16 states. One such LFSR counts in the following sequence: 0000, 0001, 0010, 0101, 1010, 0100, 1001, 0011, 0110, 1101, 1011, 0111, 1110, 1100, 1000, and then back to 0000. Note that this sequence always shifts the previous value left and computes a new right most bit. Based on this, D3 = Q2, D2 = Q1, D1 = Q0, D0 = f(Q3,Q2,Q1,Q0). Find the function f. What is the extra state not included in the counter sequence? How could you keep the counter from getting stuck in that state?
3. If the above counter were to count "down" instead of "up", what would be the functions for D3,D2,D1,D0?
4. Implement an up/down LFSR in ABEL. Use an input, UP, to count up when 1 and down when 0. Also, use a CLR input to force the state to all 0. Simulate your counter counting from 0000, up to 0110, then down to 1001, and back up to 0000. Turn in your ABEL source files, and simulation waveforms.

Rationale:

• To develop an understanding of basic sequential logic elements.
• To learn to use a hardware description language for the specification of sequential logic.