XTAL startup issues on PIC 73B
Harold M Hallikainen email (remove spam text)
I also went round and round trying to get resonators that were
guaranteed to work on a 16c74 at 16 MHz. I finally found muRata
characterizes their resonators for specific chips and frequencies (see
http://www.murata.co.jp/search/ic-e.html). Microchip could not give me
characteristics on the oscillator (gain and phase versus frequency plot),
and the resonator companies generally could not do anything without that.
One resonator company told me the resonator was fine if it worked in
their test circuit (CMOS gate with feedback resistor). The fact that they
could not supply a resonator (with internal capacitors) that worked in MY
circuit (a 16c74 with the resonator hung between two pins, center pin of
the resonator grounded) lost them a sale for a few thousand resonators.
Another issue I noticed was on one board I thought I'd "improve" it by
increasing the width of the very short traces from the 16c74 pins to the
resonator from 12 mils to 25 mils. Worked fine... 'til Microchip came out
with the 16c74b. Would not oscillate. We had to continue to use the
16c74a on that board. All my other "unimproved" boards worked fine with
So, a corollary to Murphy's law (this one from the broadcast industry)
is that amplifiers will oscillate, and oscillators won't.
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