Async communications will usually tolerate an timing error of about 0.5 bit time in 10 bits or about 5%. So if your oscillator is within 1% (10,000 ppm) you should be OK in that regard.
I suspect your serial communication problem is coming from someplace else. Maybe the radio needs real RS-232 voltages, or you have a bad interface chip or power supply for the RS-232, or you need a null modem cable that swaps the Rx and Tx data lines. On Tue, Mar 5, 2019 at 6:05 PM Joe Leikhim <jleik...@leikhim.com> wrote: > It seems some Crystal experts are on line, so here goes with my question. > > I have this microcomputer circuit (attached) that is in a Motorola > Systems Saber radio. It contains a 68HC11 uC that requires Y400 which > is a 7.3728 MHz crystal. Motorola no longer provides an OEM replacement > for this part number 4805664G32. The description is simply "XTAL > ANTIRES". -- Is that Parallel Resonant? These are those tubular 3 x 8 mm > style like the 32Khz crystals used in early watches. > > There are two critical stability requirements of this crystal. _First > _it runs the external RS232 programmer at 9600 baud. If bad the radio > cannot be read or written to, and _secondly_ the harmonics could fall on > operating channels. The network of Q403/L400 and C409 are to shift/pull > the frequency where a known harmonic might occur. -- I don't know the > normal stray load this network imparts. > > So far I have purchased some generic crystals from e-bay but they don't > seem to be solving the immediate problem which is serial communications > error. > > The crystal I removed from circuit oscillated at 7.3708 MHz per the CW > zero beat on my Icom shortwave radio. (I know, yes, it is set to WWV) > or 2 KHz low. The 10 replacement crystals sampled in at around 2.47 KHz > low. An OEM crystal that I have (the last one) 4.36 KHz low. In my > estimation, these parts should be within 30 ppm or +/- 222 Hz > > I do not know if the crystal pull network is running when I made these > measurements. It is a possibility. It is switched in and out depending > upon the radio RX frequency. I have no control over those until the > radio is read and rewritten to which I cannot in this condition. > > --------------------------- > > The E-bay generic parts: > > Frequency: 7.3728Mhz > > Frequency Tolerance: ±30ppm > > Load Capacitance: 18pF > > Mounting Type: Through Hole > > --------------------------------- > > Am I using the wrong load capacitance for the application? > > Here are my assumptions: > > CL=(24pf x 24pf)/24pf+24pf) + 6pf (stray is a guesstimate) = 18 pf > > There is not enough board space to add parts. My estimation of stray > capacitance may be high. > > I am trying to get to square one and buy the correct parts. Or maybe > this is as good as it gets and my problem is elsewhere? > > -- > Joe Leikhim > > > Leikhim and Associates > > Communications Consultants > > Oviedo, Florida > > jleik...@leikhim.com > > 407-982-0446 > > WWW.LEIKHIM.COM > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > -- --Jim Harman _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.