The output of U21, U22, U23 and U24 doesn't change at the same moment.
If a 74HCT193 counter has a typical propagation delay from "count down input" to "underflow output" of 14ns,
the output of U22 will change 14ns (37ns-23ns) later than the output of U21,
the output of U23 will change 28ns (51ns-23ns) later than the output of U21,
the output of U24 will change 42ns (65ns-23ns) later than the output of U21.
//the output of U21 changes 23ns after DN0 triggers the "count down input" of U21.
I think the 74HCT688 comparators are fast enough to notice those differences in the timing.
A change of the U21..U24 outputs will cause a change in the UNDERFLOW output... with some delay.
If a 74688 comparator has a typical propagation delay from A,B to Q of 14ns and from /E to Q of 9ns,
we are getting a response at the UNDERFLOW output after DN0 clocks U21:
U21: DN0 to UNDERFLOW = 37ns = 23ns+14ns
U22: DN0 to UNDERFLOW = 51ns = 37ns+14ns
U23: DN0 to UNDERFLOW = 74ns = 51ns+14ns+9ns
U24: DN0 to UNDERFLOW = 88ns = 65ns+14ns+9ns
In other words, when DN0 clocks U21,
37ns later the UNDERFLOW signal can't be trusted for a time window of 88ns-37ns=51ns.
Somebody please correct me if I'm wrong.
BTW: The "typical" propagation delay is different from the 'real' propagation delay,
because the 'real' propagation delay depends on temperature, output load, supply voltage...
and on what the manufacturer did when building the chip.
daniMolina wrote:
About the TOGGLE output, it was an easy fix.
WOOT.