Many thanks to the Freaks who helped out with initial questions about the accuracy of the internal RC Oscillator.
I've read AVR053 and successfully used the source code/procedure to calibrate OSCCAL.
My current question is about time-drift of the RC oscillator frequency. Reading the ATTiny85 datasheet,
I see the factory calibration of the RC oscillator is 10% and the initial OSCCAL value is burned into the signature area of the MCU. The datasheet futher states the accuracy of "User" calibration is 1% over -40 to 85C. I assume this means at a given temperature and voltage. Looking at the calibrated RC oscillator frequency versus temperature, I see the accuracy remains within 2% over 0-85C @ 5V.
Initial tests of two ATTiny85s show a factory OSCCAL of 0x55. At 25C and 5V, that turned out to be a frequency of 7.45MHz. Using the AVR053 calibration
procedure, I get an OSCCAL of 0x4C and a measured frequency of 7.97MHz.
Thus, AVR053 gets me adequate calibration. However, the difference in factory calibration and user calibration has me worried about potential time-drift of the RC oscillator. It seems that if the RC-oscillator is time-stable, then the factory calibration can be closer than 10%, more like 1-2%.
So, while it's easy to perform AVR053 calibration at time of device progamming and product creation, does anyone have any experience or knowledge about how the accuracy will remain over time? My concern is the accuracy could drift over years by more than 2% making my RS-232 transmission unreliable.
I'm comparing using a 8-pin PIC (yuk!) for the design soley for the reason the PIC datasheet shows a factory 1% accuracy at room temperature and 2% accuracy from -40-85C. This has advantage there is no worry to periodically consider recalibrating the PIC oscillator (indeed the PIC oscillator is not
able to be calibrated).
Does anyone have thoughts about the AVR RC oscillation maintaining a 1-2% accuracy over years?
Thanks once again.