tom dunigan
LPC1768 RTC frequency check with WDT. Compare RTC against MCU's microsecond Timer.
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
Diff: main.cpp
- Revision:
- 0:11cc29f22037
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp Mon Nov 16 16:56:32 2015 +0000
@@ -0,0 +1,57 @@
+// 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);
+ }
+}