Joe, I'm not Roger but I've worked a lot with Motorola micros. Your conclusion of 18pF seems reasonable as the design uses 24pF tuning caps, which are a bit high compared to the values recommended in the HC11 reference manuals. In section 2 they recommend 25pF total capacitance including strays, so I would not have used over 22pF caps to allow for tolerances. If I was going to switch from an 18pF crystal I'd consider a 15pF part on the assumption that stray capacitance is less than you allowed for. But 18 pf seems fine to me.
When measuring the oscillator be sure to use a high impedance low capacitance probe (<4pF) and only on the XTAL pin. Capacitively coupling that same probe to the EXTAL pin will give an idea how tolerant the oscillator is of additional loading. Note that Motorola recommended using only the E pin to determine oscillator frequency, it should run a 1/4 of oscillator frequency and be a very stable square wave. Jitter in E means you have a marginal oscillator. FWIW When looking at oscillator frequency it would be useful to break the line feeding C405 and bias that from an appropriate fixed supply. Bob L. > Sent: Thursday, March 07, 2019 at 1:41 AM > From: "Joe Leikhim" <[email protected]> > To: "Discussion of precise time and frequency measurement" > <[email protected]>, [email protected] > Subject: [time-nuts] Re; Motorola MC68HC11 Crystal > > Roger, does the 18 pf load, crystal I have chosen for replacement seem > correct for the design (attached)? > > Joe > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
