General Description: Why does the timing simulation in XPLA Professional give different results than simulation with the *.vho (or *.vo) file?
The VHO/VO files produced by XPLA Professional use "worst case" path timing. This file does not distinguish signal paths through the PAL from signal paths through the PLA. If an equation has more than 5 product terms (4 for the XPLA2), the timing in the VHO file will show PLA timing for all signal paths in the equation.
The timing simulator in XPLA Professional uses the actual path timing. Therefore, if the signal path is through a product term in the PAL, the PAL timing is used. If the signal path is through a product term in the PLA, the PLA timing is used.
Therefore, differences can be seen in the two methods of timing simulations. This can show up in path delays using product term clocks as well. The XPLA Professional timing simulation uses circuit details and is a more accurate representation of the actual, worst-case device timing.