SN74HC4040: technical consulting

Hello,

The issue they are running into is inherent to SPICE simulation -- flip-flops, latches, and registers create metastable states that are not easily expressed by a simulator. Sometimes this kind of circuit can be successfully simulated, but in my experience, it is pure luck which circuits will work and which will not. Most will produce singular matrix solutions to the node voltage equations and will fail.

I would recommend to build a prototype rather than simulate with this type of device -- or use a simulator that includes a built-in digital simulation capability to handle the latching logic devices. Unfortunately, there is no reliable universal digital simulation method that also shows the analog behavior of the devices.