In AN-B-020, the procedure to trim the 16MHz crystal is described. This procedure uses either a high speed counter or a spectrum analyser. This seems to be the recommended method for crystal calibration.
In UM-B-008, the 'xtrim cal' procedures is described. This procedure uses a 500ms pulse applied to one of the pins to do automatic calibration.
Is the 'xtrim cal' method allowable for trimming the 16MHz crystals on the production line? If so, what accuracy is required for the 500ms pulse? Can this process be used even if we intend to use an external EEPROM to hold the xtrim values and the BD address (as described in AN-B-023)?
Thanks for any help you can give.
Hi,
We actually recommend the 'xtrim cal' method in production. It is much easier to handle. The accuracy of the applied pulse dermines the accuracy of the trimming, so if you want to trim to within 2ppm, your pulse needs to be within that limit as well.
You would have to modify the production test application to store the trimmed value in the external eeprom, but other than that everything should be pretty straight forward.