Slide 42 of 93
VHDL allows the designer to describe systems at various levels of abstraction. As such, timing and delay information may not always be included in a VHDL description.
A delta (or delta cycle) is essentially an infinitesimal, but quantized, unit of time. The delta delay mechanism is used to provide a minimum delay in a signal assignment statement so that the simulation cycle described earlier can operate correctly when signal assignment statements do not include explicitly specified delays. That is:
1) all active processes can execute in the same simulation cycle
2) each active process will suspend at wait statement
3) when all processes are suspended simulation is advanced the minimum time necessary so that some signals can take on their new values
4) processes then determine if the new signal values satisfy the conditions to proceed from the wait statement at which they are suspended