For accurate time measurement projects using microcontrollers, I need to specify the accuracy of my meter.
Since it is based on crystals or crystal oscillator, the accuracy of the oscillator plays major part in defining specifications.
Looking at the datasheet of a crystal oscillator http://www.farnell.com/datasheets/520.pdf I found 2 terms:
1. Standard frequency stability over operating temperature range. (+- 100ppm) Is this 100 ppm per degree or is it per whole temperature range as said?
2. Frequency tolerance at 25 degrees C. (+-25ppm) how would this affect the tolerance at other temperatures?
From these 2 parameters How do I specify the accuracy of my time measurement over the whole working temperature range and to see weather normal crystal oscillator satisfies my accuracy requirements or do I need TCXO (Temperature Compensated Crystal Oscillator)?
What other errors should be included from the microcontroller side like the length of the counter, no of clocks per cycles fixed or floating point math routine errors etc?
[Edited] I need to measure the speed of a moving object so I use a slotted IR transmitter receiver and attached a 2mm diameter plate to go in the slot. I measure the time it took to cross the sensors and from this I calculate and display the speed.
Is this method acceptable (taking into account the rise and fall time of the optical receiver) or is there a better way?
Mahmood