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Your design goals for accuracy, repeatability, set approach and HW you use
to determine design performance. A xtal is most common approach. The next
step up would be to use a precision external osciullator. For even better results
the following is very effective.
One approach to getting very high accuracy is at production test. Basically you
have a routine inside of PSOC that talks to a high precision external freq counter
and feed an onboard PSOC clk to a pin for the external counter to read. GPIB, USB, UART
commonly used to external counter. PSOC drives a testbed heater/cooler, and records
the external counter and onboard die temp values building a table for generation of
the coefficients to a power fit equation. The table or power curve equation then become
the means for interpreting any value read by PSOC timer/counter. This essentially takes
device to device variation out and corrects for temp. Its only drawback is it does not fix
component aging, but that usually is the lesser problem, or can be characterized and
compensated for. Sounds complicated but really quite simple.
The attached pic shows general idea, in this case replace the precision ref source with
a freq cntr. This pic shows approach to correct A/D and Reference accuracy of a PSOC.
This approach is general.
Regards, Dana.
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