[time-nuts] Motorola MC68HC11 Crystal

[Jim Harman]

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 <jleikhim at 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
>
> JLeikhim at Leikhim.com
>
> 407-982-0446
>
> WWW.LEIKHIM.COM
>
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-- 

--Jim Harman