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32Khz crystal accuracy test

John Pillai
Intellectual 550 points

I need to know practical performance of the 32khz crystal drift over the period of time. hence I   did a prototype which  configures a timer with 32khz clock in pwm mode for a period of 2 sec I see a drift of around 115microsec . I have read the drift should be only around 20ppm. is there any way to know the  expected drift  for the 32 Khz clock  also any better approch of experimenting this paratically !!   I am using Beagleboard Xm and evm3730.

Thankyou!!

  • Intellectual 695 points
    Hi John
    Given that you're using a 32KHz, it'd be also useful to check the signal itself through an oscilloscope, just keep in mind that the probe will load a little bit the crystal terminals so that is going to affect the accuracy as well. Using a low load probe can solve that.
    On the other hand you method should be fine, I just wonder how much time of that 2 s period is "processing time", thus, time the system uses to toggle the pulse, make the PWM, timers, etc
  • in reply to Mario Hernandez
    Intellectual 550 points
    hello Mario,

    Sorry i Missed how I was measuring. I tapped the PWM pin using oscilloscope and was taking readings manually(tried max to be accurate enough).I will check on the low load probe as you suggested.And how do i find out that processing time of system that u mentioned?can that be benchmarked or it will vary.
  • in reply to John Pillai
    Intellectual 695 points

    Generally, you can check the datasheet or the instructions set section. There it is specified how many cycles any instruction takes to complete, then if, let's say you wrote and Interrupt Routine plus a PWM bit banging, try to identify how many instructions were generated in assembly code and quantify the time they take to complete. Another way, a lot more effortless, is using the debugger to measure times in completing such tasks.

    That way, you can have an approximate idea on how much processing time you have to account for you time calculations