tom dunigan / Mbed 2 deprecated k64f_rtcisr

measure RTC crystal freqeuncy against MCU crystal

Dependencies:   mbed

This program uses the RTC 1-second interrupt to capture the Timer microsecond value to estimate the error in the RTC's 32KHz crystal. (the K64F MCU's crystal could be in error as well, though mine was within 20 ppm). This program measured the error at about 250 ppm, well out of the 20ppm spec for the RTC crystal???

main.cpp@0:b30b04f4a2aa, 2015-10-27 (annotated)

Committer:
manitou
Date:
Tue Oct 27 21:07:11 2015 +0000
Revision:
0:b30b04f4a2aa
first cut

Who changed what in which revision?

UserRevisionLine numberNew contents of line
manitou 0:b30b04f4a2aa 1 // k64f rtcisr
manitou 0:b30b04f4a2aa 2 // seconds interrupt from RTC compare micros() to check RTC frequency
manitou 0:b30b04f4a2aa 3 #include "mbed.h"
manitou 0:b30b04f4a2aa 4
manitou 0:b30b04f4a2aa 5 #define micros tmr.read_us
manitou 0:b30b04f4a2aa 6
manitou 0:b30b04f4a2aa 7 DigitalOut led(LED_RED);
manitou 0:b30b04f4a2aa 8 Timer tmr;
manitou 0:b30b04f4a2aa 9
manitou 0:b30b04f4a2aa 10 volatile static unsigned long ticks,us;
manitou 0:b30b04f4a2aa 11 static unsigned long us0=0;
manitou 0:b30b04f4a2aa 12
manitou 0:b30b04f4a2aa 13 extern "C" void RTC_Seconds_IRQHandler() {
manitou 0:b30b04f4a2aa 14 if (us0 == 0) us0 = micros();
manitou 0:b30b04f4a2aa 15 else {
manitou 0:b30b04f4a2aa 16 ticks++;
manitou 0:b30b04f4a2aa 17 us = micros() - us0;
manitou 0:b30b04f4a2aa 18 }
manitou 0:b30b04f4a2aa 19 led = !led; // toggle led
manitou 0:b30b04f4a2aa 20 }
manitou 0:b30b04f4a2aa 21
manitou 0:b30b04f4a2aa 22 int main(){
manitou 0:b30b04f4a2aa 23 int ppm, tprev=0;
manitou 0:b30b04f4a2aa 24
manitou 0:b30b04f4a2aa 25 tmr.start(); // for micros
manitou 0:b30b04f4a2aa 26 time(NULL); // start RTC
manitou 0:b30b04f4a2aa 27 RTC_IER = 0x10 ; //TSIE enable seconds interrupt
manitou 0:b30b04f4a2aa 28 NVIC_EnableIRQ(RTC_Seconds_IRQn);
manitou 0:b30b04f4a2aa 29 while (true) {
manitou 0:b30b04f4a2aa 30 ppm = 1000000*ticks - us;
manitou 0:b30b04f4a2aa 31 ppm = 1.e6 * ppm/ (float)us;
manitou 0:b30b04f4a2aa 32 printf("%d %d %f %d\n",ticks,ticks-tprev,us*1.e-6,ppm);
manitou 0:b30b04f4a2aa 33 wait(2.0);
manitou 0:b30b04f4a2aa 34 }
manitou 0:b30b04f4a2aa 35 }