All Nitron MCU´s have an internal oscillator, so they can run without an
external clock. The internal oscillator runs at a 12,8 MHz, giving a busfrequency
of 3,2 MHz. The internal frequency can vary by ± 25%. You can trim
it to ± 0,4% of 3,2 MHz by writing the right value in the OSCTRIM
register at adress $38. Normally the value from adress FFC0in FLASCH memory
is copied into the OSCTRIM register. In Monitor Mode this happens automatically,
in User Mode you must take care of this within your program. The problem
is to get the right trim value for the oscillator. In Application Note
AN2312 Motorola shows how this can be done. You can download the software at
Motorola after registering. The application note describes two methods
for callibration. Both methods need an external reference frequency. The
first method uses a power line as reference clock. This is a little tricky
to do, the other method is much easier. If you take a look at the schematics
to send the reference frequency via RS232 you can see it is quite similar
to the programming circuit.
You can use your programmer to calibrate the oscillator. Take care not
to enter Monitor Mode. This can be done by not putting all the signals
in the right state or removing the external oscillator. After you have
done this, you can use Hyperterm to make a connection with 9600 Baud to
the controller. Then send as many @ signs until the right value is found.
If the right value is found the MCU signals this by toggeling PTA 3. You
can check this by putting a LED to PTA3 or by checking the voltage at PTA3,
which then should be ca. 2.5V.