VHDL Programming

If only the first value is specified, the signal will cause a trigger when

that value is reached on the signal. When both values are specified, the

signal must transition from the first value to the second value to cause a

trigger. In Figure 18-13, signal current_statemust transition from state

reset5to state reset6to cause a trigger.

Complex Triggers

Watchpoints and breakpoints can be combined to create more complex

triggers. Complex triggers allow the designer to zoom in on exactly the

area of the design to capture data from, or to locate more precisely the

cause of a design error. For instance, to specify exactly the case when the

instruction is written from the memory bus to the instruction register

after state reset6, the following watchpoints can be specified. Place a

watchpoint expression of reset6on signal current_state, and a watch-

point expression of 1 on signal ready. When both of these expressions are

410 Chapter Eighteen

Figure 18-10
Trigger on Loadi2
and Captured Data
Displayed.