Constantin
Well-known member
Hi all,
tonight, I though I'd take a crack at figuring out how to calibrate my RTC using a PPS signal from a GPS receiver. The first step was calculating by how much the MCU clock seems to be off. In this particular instance, about 14.5PPM, BTW. Then wait 10 seconds (as defined by seconds() command from the time library) and see how many µs that took using the MCU as a yard stick. Then multiply the MCU offset (in PPM) by interval(µs)/(time measured by MCU while RTC ticked). Usually the latter is less than 1PPM off relative to the MCU clock, and the multiplier is something like 0.99999 etc.
The product is then used to calculate the offset for the compensate function (about -124 in this case). However, setting this number seems to have no effect. I'm sure there is something obviously wrong with my code, but I'm missing it. Any help would be appreciated!
tonight, I though I'd take a crack at figuring out how to calibrate my RTC using a PPS signal from a GPS receiver. The first step was calculating by how much the MCU clock seems to be off. In this particular instance, about 14.5PPM, BTW. Then wait 10 seconds (as defined by seconds() command from the time library) and see how many µs that took using the MCU as a yard stick. Then multiply the MCU offset (in PPM) by interval(µs)/(time measured by MCU while RTC ticked). Usually the latter is less than 1PPM off relative to the MCU clock, and the multiplier is something like 0.99999 etc.
The product is then used to calculate the offset for the compensate function (about -124 in this case). However, setting this number seems to have no effect. I'm sure there is something obviously wrong with my code, but I'm missing it. Any help would be appreciated!
Code:
//This sketch attempts to calibrate the RTC crystal on a Teensy 3
//using a PPS signal from a GPS receiver
//enables RTC on Teensy 3:
#include <Time.h>
//determine what Teensy 3 pin the PPS Signal goes to:
const byte PPS = 14;
void setup() {
pinMode(PPS,INPUT); //PPS Signal input
Serial.begin(115200);
// set the Time library to use Teensy 3.0's RTC to keep time
setSyncProvider(getTeensy3Time);
while (!Serial); // Wait for Arduino Serial Monitor to open
delay(100);
if (timeStatus()!= timeSet) {
Serial.println("Unable to sync with the RTC");
}
else {
Serial.println("RTC has set the system time");
}
}
void loop() {
if (Serial.available()) {
time_t t = processSyncMessage();
if (t != 0) {
Teensy3Clock.set(t); // set the RTC
setTime(t);
}
}
//how many seconds is the time interval long over which the
//MCU and RTC will be measured:
int time_interval = 10; //seconds
Teensy3Clock.compensate(-124);
//now determine how many microseconds the MCU takes to count to x
unsigned long time_interval_mcu = mcu_time_interval_calc(time_interval);
//now determine how many microseconds the RTC takes to count to x
unsigned long time_interval_rtc = rtc_time_interval_calc(time_interval);
long PPM_delta_MCU= time_interval_mcu-1000000L*(long)time_interval;
Serial.print("i.e. ");
Serial.print((float)PPM_delta_MCU/(float)time_interval,2);
Serial.print("PPM for the MCU and ");
long PPM_delta_RTC= time_interval_rtc-1000000L*(long)time_interval;
Serial.print((float)PPM_delta_RTC/(float)time_interval,2);
Serial.println("PPM for the RTC.");
//because the combination of both formulas cancel out the interval time, omit
float RTC_cumulative_PPM = (float)PPM_delta_MCU*(float)1000000/(float)time_interval_rtc;
Serial.print("Taken together, combined RTC offset appears to be ");
Serial.print(RTC_cumulative_PPM,2);
Serial.println("PPM for the RTC.");
int RTC_compensate =(int)(-RTC_cumulative_PPM/.1192);
Serial.print("This suggests a ");
Serial.print(RTC_compensate);
Serial.println(" rtc_compensate factor...");
}
time_t getTeensy3Time()
{
return Teensy3Clock.get();
}
/* code to process time sync messages from the serial port */
#define TIME_HEADER "T" // Header tag for serial time sync message
unsigned long processSyncMessage() {
unsigned long pctime = 0L;
const unsigned long DEFAULT_TIME = 1357041600; // Jan 1 2013
if(Serial.find(TIME_HEADER)) {
pctime = Serial.parseInt();
return pctime;
if( pctime < DEFAULT_TIME) { // check the value is a valid time (greater than Jan 1 2013)
pctime = 0L; // return 0 to indicate that the time is not valid
}
}
return pctime;
}
unsigned long mcu_time_interval_calc(int time_interval)
{
unsigned long time_begin, time_end, delta;
while(digitalRead(PPS)==LOW){
} //wait until it goes high but ignore the first one
delay(200); //delay 200ms to let PPS signal die
while(digitalRead(PPS)==LOW){
} //wait until it goes high, second time is a charm
time_begin=micros();
for (int i=0;i<time_interval;i++)
{
delay(200); //delay 200ms to let PPS signal die
while(digitalRead(PPS)==LOW){
} //wait x rounds
}
time_end=micros();
delta=time_end-time_begin;
Serial.print("Over a ");
Serial.print(time_interval);
Serial.print(" second interval, the MCU took ");
Serial.print(delta);
Serial.println(" 'MCU micro-seconds'.");
return (delta); //return the interval that the MCU will run
}
unsigned long rtc_time_interval_calc(int time_interval)
{
unsigned long time_begin, time_end, delta;
byte current_second=second();
while(current_second==second()){
} //wait until it changes
time_begin=micros(); // start clock
current_second=second();
for (int i=0;i<time_interval;i++)
{
while(current_second==second()){
} //wait x rounds
current_second=second();//each time increment the seconds
}
time_end=micros();
delta=time_end-time_begin;
Serial.print("Over a ");
Serial.print(time_interval);
Serial.print(" second interval, the RTC took ");
Serial.print(delta);
Serial.println(" 'MCU micro-seconds'.");
return (delta); //return the mcu us the RTC took
}
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