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Actually my question would be how big of a gap in term of precision between internal oscillator and a resonator at 4 MHz?
I am wondering if I can just remove the external part, save some space and the cost of the part.
I don't need a long term calculation such as an alarm clock, I am just calculating distance using ultrasonic and I don't need precise distances. I only need to compare distance differences in units as long as during my comparison my units are identical.
Mike
It all depends on the device. Some of the old 16F parts weren't very accurate at all. Most all the 18F parts have better than 1% accuracy - even over a wide temperature range. Your datasheet will tell you.
lilimike,
For example, a PIC18F4550 has a +/- 2% internal frequency variation at room temperature (25 deg. C). This is according to the datasheet, electrical characteristics section. However, these chips are calibrated in the factory and the 2% is just the worst case scenario. The accuracy in frequency is ussually pretty good.
A typical 4MHz crystal can have a frequency tolerance of +/- 30 ppm at room temperature. This is equivalent to +/- 0.003% in frequency variation.
For the type of application that you want, the internal oscillator should be good enough.
Robert
I am using a 16F628A.
I did see the datasheet but I was not sure exactly how to interpret it.
Even with temperature change as long as there is no drastic change within a few seconds I should still be ok.
Thank you for the information.
Mike
The next sentence is from the 16F628A datasheet page 1.
Check page 153 of the datasheet for the graph "TYPICAL INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE, VDD = 5 VOLTS".Quote:
Precision internal 4 MHz oscillator factory
calibrated to ±1%
Robert
yes I see, looking at the graph is more clear.
Thank you