Page 152 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

State Diagrams, State Tableg and State Machines 7 33

The states are represented by the circles labeled 0-cents, 5-cent5, 10-cents,
15-cents, and ZO-cents, and the values on the dispense and change outputs
are associated with these states. The arcs connecting the states are called
state transitions and the values of the nickel and dime inputs associated with the
state transitions are called guard conditions. The controller will only sequence
between two states if the values on the nickel and dime inputs match the guard
conditions.
Let’s assume that the controller is in its initial state of 0-cents. The values
of the nickel and dime inputs are tested on every rising edge on the clock.2 As

long as no coins are deposited, the nickel and dime inputs remain at 0 and the
controller remains in the 0-cents state. Once a coin is deposited, the next
rising edge on the clock will cause the controller to sequence to the 5-cents
or the IO-cents states depending on the coin’s type. It is at this point that the
controller sends an acknowledge signal back to the receiver instructing it to reset
the nickel and dime signals back to 0 and to await the next coin.
Note that the 0-cents, 5-cents, and 10-cents states have state transitions
that loop back into them (the ones with associated nickel = 0 and dime = 0
guard conditions). These indicate that the controller will stay in whichever
state it is currently in until a new coin is deposited.
So at this stage of our discussions, the controller is either in the 5-cents
or the 10-cents state depending on whether the first coin was a nickel or dime,
respectively. What happens when the next coin is deposited? Well this depends
on the state we’re in and the type of the new coin. If the controller is in the
5-cents state, then a nickel or dime will move it to the 10-cents or 15-cents
states, respectively. Alternatively, if the controller is in the 10-cents state, then
a nickel or dime will move it to the 15-cents or 20-cents states, respectively.
When the controller reaches either the 15-cents or 20-cents states, the
next clock will cause it to dispense a “gizmo” and return to its initial 0-cents
state (in the case of the 20-cents state, the controller will also dispense a nickel
in change).

2 The controller is known to sequence between states only on the rising edge of the clock, so
displaying this signal on every state transition would be redundant.