THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

STATE MACHINES

by Ingo Cyliax

Start ý An ASM Example ý Input and Output Signals ý Implementation ý One-Hot Encoding ý Synthesize ý Initializing ý Until Next Time ý Sources and PDF

IMPLEMENTATION

At this point I have tied down the input and output signals, but havenýt decided how to implement the actual state machine. Traditionally, there are several ways you can implement state machines. State machines are usually like the one shown in Figure 3. There is a state variable, which is implemented as registers. There is a next function generator, which will look at the current state of the machine, evaluate some inputs, and compute what the next state should be. And, there is an output function generator, which computes the output based on the current state and input signals.

Figure 3ýA typical state machine has a state variable register and logic or look-up tables that compute the next state as well as outputs based on the current state and inputs from the outside.

The function generators can be look-up tables, logic gates to generate the output function and next state function. The state variable contains some kind of encoding of the states in the state machine. One type of encoding would be to assign a sequential binary number to all of the states in the machine and use the binary number as the representation in the state variable register. The advantage of doing this is that you can represent a large number of states with log2(n) register bits. The disadvantage is that, for some state transitions, you may have a large number of bits changing in the state variable register.

Generally, you would encode the states in such a way that only a few bits change in the register. This reduces power consumption and makes the next state equations smaller because, on average, only a few bits are affected by a state change. Encoding methods such as gray codes are a favorite for these types of state machines.

PREVIOUS • NEXT