State Machine Design Example

Tom Kelliher, CS 240

Mar. 3, 1999

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Homework due Friday. Homework questions? Solution will be posted by Monday.

Review Monday. Midterm Wednesday. Friday?

Assignment

From Last Time

State machine model. Clocks and latches.

Outline

1. Flip-Flops.

2. State machine design example.

Coming Up

MIPS datapath and control.

D Flip-Flop

An edge-sensitive D flip-flop with a falling-edge trigger:

Trace operation.

The importance of set-up and hold times.

Bicycle Light Design Example

The product: a bicycle light which has a button and a number of LEDs. Push the button once and the LEDs shine steadily. Push again and it goes off. Push a third time and the LEDs flash on and off a few times a second. Push one more time and it goes off again.

Design steps:

1. Determination of inputs and outputs.

2. Determination of machine states.

3. Transition assignment.

4. State assignment.

5. Truth tables.

6. Implementation.

Questions along the way:

1. How many state bits?

2. The state assignment problem. In this case: simplify LED equation.

3. What to do with unused states? Don't cares; or do we care?

4. What clock frequency? Issues: design simplification, power consumption.

5. An alternative: one-hot state machines.