At 17:54 09.01.2003 -0800, you wrote:
Update: I found the problem. I was using Pericom's 74FCT521 with TI's 74HCT164's. I changed it all over to TI's HCT devices and it works now. It's kinda odd though, I always thought FCT and HCT devices would work together.They should, yes.
I'm not sure if this was the real cause of your fault. I think this is a typical setup&hold problem. If your combinatorial input to the shift register is not stable for so-and-so many nanoseconds before and after the rising edge of the clock signal, you get nearly random results. This is true for all kinds of flipflops - make sure that the input signal does not change for a time window around the rising edge of the clock signal, and all your problems are gone.
Here is my next problem though... when I power on the circuit, there is a resistor/cap on the clear line of the '164's. This resets the outputs to low on power on. When you power on the circuit with a high input to one of the channels, the analyzer is stuck with = high and never goes low. I'm thinking I need to apply one clock cycle to the '164 on power up, and after the power on reset???The '164 has an async clear, independant of the shift input(s), and independant of the clock line. No idea what might cause this effect. Checked all powers? Caps near parts? "ringing" or overshoot on clock lines? Anything else causing glitches?
any ideas?
ciao,
--
Jens Sch�nfeld
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