Observe that the Set OR Hold 1 equation has been modified to include the external input
condition that may occur for each set or hold 1 transition. If an external input condition occurs
for a set or hold 1 transition, the condition must be ANDed, hence the “?,” with the PS expression.
If an external input condition such as SHORTEN or SHORTEN does not appear beside a
transition line in a state sequence or state diagram, then the external input condition in the Set
OR Hold 1 equation is simply ignored, because SHORTEN 1 SHORTEN 5 1, which indicates
that no external input condition is required to change to the next state.
The D excitation equations can be written by inspection using the state sequence diagram
or the state diagram for the design.
Once the D excitation equations are obtained, you can draw the complete circuit diagram
for the complex state machine. You can also use the D excitation equations to write the VHDL
code for the design. If the state diagram has Moore or Mealy outputs, you must also write the
equations for these outputs and include the equations in the circuit diagram and the VHDL code.
The disadvantage of using the algorithmic equation method for D flip-flops compared to
using the two-process PS/NS method is the hassle of obtaining the correct D excitation equations
and drawing the circuit diagram.
Figure 9.10 shows a modified form of the state diagram in Figure 9.9. Observe that the
flip-flop output signals rather than the flip-flop output signal values are place in each state,
which provides a more compact state machine design description—especially for larger one-hot
encoded designs.