I've discovered two more things, but not resolved anything yet...

This from the Microchip Knowledge Base, ticket 1-10799

"If the device has an internal RC oscillator, and (possibly) an OSCTUNE register but 
no OSCCAL register, it has most likely been pre-calibrated at the factory before 
shipping and you do not need to do anything to calibrate it - you are however 
permitted to tweak the frequency using the OSCTUNE register when running your 
code.  To make sure your part works this way, read about the oscillator design in 
the data sheet."

- OK, the PIC16F88 has an OSCTUNE register but no OSCCAL register, therefore 
the calibration is done in the factory and I am not required to do anything to 
calibrate it. Cool, guess that's why the data sheet is so brief. Therefore some of my 
parts must be out of spec, or at least extremely close to being out of spec 
(hedging!).

Also discovered the following, from "PIC16F87/88 Rev. B1 Silicon Errata", 
DS80171K...

Issue 3. Module: Internal RC Oscillator (on page 3):

"...it is possible for the oscillator to overshoot the selected
frequency:..."

"When it is required for the application to run at
8 MHz, it is recommended that the application
does not start executing code at 8 MHz until the
60 ms firmware delay (see issue 2) has
completed. During the 60 ms settling period, the
application can execute code up to 4 MHz. Upon
completion of the 60 ms firmware delay, the 8 MHz
can be selected via the IRCF bits."

- OK, so I was immediately selecting 8MHz after reset. Changed this now so 
immediately select 4MHz, wait 60ms, then select 8MHz. No change in running 
requency. Have tested this with only one chip so far.

-- 
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: 027 433 4069
eMail:  brent.brown@clear.net.nz


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