MC68HC908QB8 & 32.768kHz Crystal

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MC68HC908QB8 & 32.768kHz Crystal

Contributor II
Has anyone any experience using a 32.768Khz Crystal with the 980QB8 processor?
The main problem I have is consistency.  I can get the device to oscillate but the capacitors seem to vary significantly between devices.

What Crystal, capacitors, feedback and limit resistors did you use.

Alban edit: change subject unit from "Mhz" to "kHz"

Message Edited by Alban on 2007-06-06 10:53 AM
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Specialist III
Do you mean that you need to vary the capacitor values to achieve oscillation?  If so, it may indicate that you need to reduce the "limit" resistor in series with the crystal.
A good starting point for the capacitors would be 22p for each, although to achieve the correct operating frequency may need somewhat different values.  The shunt resistor used for biasing should have a value of at least 1M0, but an even higher value is probably preferred for the low frequency crystal.
The series resistor might commence with a value of say 100k, but may need reduction if oscillation is reluctant to start.
Be aware that the low frequency crystal may take a second or two to achieve maximum oscillation amplitude.  This will need to be taken into account before switching from the internal oscillator.
I don't know the practicality of using such a low frequency crystal for the QB8 device - this will result in a bus frequency of only 8.192 kHz, so operation will be very very slow.  At this frequency, many of the peripherals would not be usable.
A better choice may be to use a device containing a PLL or FLL facility to multiply the bus frequency.  The 9S08QG8 might be a possibility.

Message Edited by bigmac on 2007-06-07 01:15 AM
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