LogiSim

Decided to do a LogiSim for the design. About a day to put together the design and another day to debug the design:

The main problems discovered were:

Incorrectly modeled 74161 and 74173 in the library. This was a real pain because the models have strange behavour only in certain circumstances. In the case of the 74161 I fixed the problem, in the case of the 74173, I swapped it for a 74161 (just to get the simulation to work).

Another problem with the simulation is that tri-state logic often causes the simulation to start properly. The was fixed by using pull-up/down resistors.

Finally, my schematic looks good (just as well as the PCBs arrived this morning). The main issue is the Carry LED is rather confusing (I am not going to solder it in).

From a programming point of view, only holding the Carry for one extra cycle works for ADD but not for the composite SUB (i.e. NAND -1; ADD ?; NAND -1;). May be better to swap XOR for SUB in the instruction set.

Otherwise the simulation works as expected. So I have confidence that the PCB will be good.

AlanX