Instability in my 74x4040 implementation

[chyvonomys]

Hi. Library was lacking this chip so I tried implementing it using JK triggers (a) and three 74x161 (b). Variant (b) is not stable: QA is randomly 1/0 when I hit play button. Why is it so? How do I fix this?

[hneemann]

I suspect that there is a race condition between the two inverters.
If they do not switch at the same time, it may happen that 1 or 0 is shown at QA, depending on which inverter is faster.
Either there is a CLR first and then a CLK or vice versa.

[mengstr]

The variant with 74161 is not a good representation of the 4040 anyways. The 4040 is a ripple counter and the 7416x-series are synchronous counters which makes them behave differently.