piclist 2002\10\11\090552a >
Thread: General Serial Tips
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face BY : Russell McMahon email (remove spam text)



> > > >Russell McMahon wrote:
> > > > > A ceramic resonator has ample accuracy to guarantee the required
> > > > > accuracy if  used at both ends.

> > > >   A ceramic resonator *might* have the accuracy to guarantee if used
at
> > > >both ends. Is much better if slow rate are used.

SUMMARY:        Ceramic resonators from reputable manufacturers are very
easily able to meet the timing requirements typical of asynchronous
communications.

DETAIL:                As I showed previously, the required timing accuracy
for a typical single byte in asynchronous communications is slightly over
5%. This is the sum of the errors allowed at both ends. If each end has a
worst case timing error below +/- 2.5% the link will work. This applies to a
single byte sent at ANY communications speed, be it 110 baud or 38400 or
higher. As the link is resynchronised on a character by character basis a
+/- 2.5% accuracy is adequate although higher accuracy is not at all
harmful.

Typical lifetime accuracies of ceramic resonators from reputable
manufacturers are in the order of 1% worst case including initial,
temperature and lifetime drift errors. This is assuming that the worst case
occurs in the same direction for each error source. Even then such a
resonator would be about 2.5 times more stable than necessary for proper
asynchronous connection timing.

Conceivably one may be able to obtain suitably inferior no-name resonators
that do not meet the requisite +/- 2.5% requirements.


               Russell McMahon

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