I'd like to use a ATTiny25 MCU to sending RS-232 output using the internal RC oscillator. I'm familiar with choosing clock crystals with zero error-rate divisors as well as using a 32768Hz crystal for calibrating the RC oscillator using the OSCCAL register.
However, I'd like to have reliable RS-232 communication without any external reference or calibration. I think 2% timing accuracy is sufficient from my application. Reading AVR053, I see that there are multiple RC oscillator versions. From the source code that accompanies AVR053, the ATTinyx5 series uses the 5.0 oscillator.
I've seen some data suggesting that AVR's have an initial RC accuracy of +/- 10-20%, but the RC oscillator can be tuned to 1%.
Examining the datasheet for the ATTiny25, it appears the RC oscillator will be accurate enough for my application without any calibration. Specifically, "Figure 24-35. Calibrated 8 MHz RC Oscillator Frequency vs. Temperature" shows that at 5V, the RC frequency ranges from 7.86MHz at 0C to 8.09MHz at 80C. That's within the 2% specification and should be fine for RS-232 transmission.
My question is how much to believe the ATTiny25 graph. While the graph shows a range of 7.86-8.04MHz over a wide temperature range, are those frequencies subject to that +/- 10-20% accuracy figure quoted elsewhere. If not, what is the accuracy of the Tiny25 compared to the datasheet graph. I can't find a general statement of percentage accuracy of the RC oscillator in the ATTiny25 datasheet.
I've heard that in general PICs have a more accurate RC oscillator. Looking at a 8-pin PIC datasheet, it just shows a +/- 2% accuracy of the RC filter over the 0-85C temperature range -- roughly the same as the ATTiny25 graph in the datasheet.
Thanks for any thoughts.