tom dunigan / Mbed 2 deprecated driftrtc

LPC1768 RTC frequency check with WDT. Compare RTC against MCU's microsecond Timer.

Dependencies:   mbed

Check LPC1768 RTC crystal frequency using WDT. Measure with MCU's microsecond Timer. The MCU's crystal may be off a bit as well. I measured the RTC frequency error at 22 ppm. The spec for the RTC crystal is 20 ppm. Using NTP and a GPS, I had measured the MCU's crystal frequency error at -2 ppm.

Frequency measurements for other MCU/RTC cyrstals at

https://github.com/manitou48/crystals/blob/master/crystals.txt

main.cpp

Committer:
manitou
Date:
2015-11-16
Revision:
0:11cc29f22037

File content as of revision 0:11cc29f22037:

// use WDT to measure drift of RTC and IRC
// seems WDT ISR will only fire once
#include "mbed.h"

DigitalOut led1(LED1);
uint32_t ticks;

void kick() {
    __disable_irq();
    LPC_WDT->WDFEED = 0xAA;
    LPC_WDT->WDFEED = 0x55;
    __enable_irq();
}

void isr_wdt(void) {
    ticks++;
    led1 = 1;
    LPC_WDT->WDMOD |= (1<<3);
 //    LPC_WDT->WDMOD &= ~(1<<2);
 //   kick();
}

void wdt_init() {
    LPC_WDT->WDCLKSEL = 0x2;               // Set CLK src 0 IRC  1 PCLK   2 RTC
   // uint32_t clk = SystemCoreClock / 16;    // WD has a fixed /4 prescaler, PCLK default is /4
    LPC_WDT->WDTC = 0xfffffff;    // max countdown
    LPC_WDT->WDMOD = 0x01;  // enable but no reset
    NVIC_SetVector(WDT_IRQn, (uint32_t)&isr_wdt);
    NVIC_ClearPendingIRQ(WDT_IRQn);
    NVIC_EnableIRQ(WDT_IRQn);
    kick();
}

int main() {
    uint32_t t,t0=0,us,us0,prev;
    int ppm;
    Timer tmr;
    tmr.start();
    printf("hello %x\n",LPC_WDT->WDMOD);
    wait(1.0);
    wdt_init();
    prev=LPC_WDT->WDTV;
    while(1) {
        t=LPC_WDT->WDTV;
        us = tmr.read_us();
        if (t0==0 || t0 == 255) {
            t0 = t;
            us0 = us;
        }
        double tf = (t0-t)/8192.;  //  /8192 for rtc   1000000 for IRC
        double secs = 1.e-6*(us-us0);
        ppm = 1.e6*(tf-secs)/secs;
        printf("ticks %d %d %u %d us  %x %d ppm \n",ticks,prev-t,t0-t,us-us0,t,ppm);
        prev=t;
        wait(2.0);
    }
}