Hi all,
I am building a simulation of an old 70s design based on 74161s and I have had a lot of trouble getting a set of counters to operate according to my expectations.
In my search for understanding more I discovered a recent document that had identified an error in the 74161 multisim model related to the counter executing on falling edge of clock, should be rising edge . Unfortunately it did not solve my headache.
Essentially I have three 74161s in cascade - the RCO for the first counter nicely carries over via ENT to trigger the second counter (ENP is high). But, when the second counter's RCO carries over to the third counters ENT it stays high for a long period of time (the duration of the QA) and sets off the third counter to "race at clock speed" similar to the first counter. When I do the same thing but instead carry over to ENP (having ENT high all the time) the counters work as expected.
The old schematics I use as a base, as well as all cascade counter reading I have come across indicates RCO should connect with ENT and not ENP – then again reading the 74161 datasheet it states that RCO stays high during the duration of QA.
Question to the forum: is the model for 74161 in multsim even more incorrect than the clock edge issue or is my “three cascaded counter” design incorrect? Please find attached the multisim model.
Unfortunately my multisim file is saved as .ms13 which is not allowed for forum upload, so I have attached a jpg of schematics and simulation results instead.
I appreciate any insights.
br /mattias