Milliseconds using square wave RTC

Question

i am trying to make a code that prints the time hh:mm:ss:ms using RTC,i have made it using millis() but it is not accurate +/-2 milliseconds

ho can i make it using sq wave and intterupt

here is the code that i have used

#include <Wire.h>
#include "RTClib.h"
RTC_DS1307 RTC; // This is the DS1307 hardware RTC
RTC_Millis SoftRTC; // This is the millis()-based software RTC
long startMS; // This will hold the start time in milliseconds
void setup ()
{
 Wire.begin();
 RTC.begin(); // Connect to the DS1307
 SoftRTC.begin(RTC.now()); // Initialize SoftRTC to the current time
 startMS = millis(); // get the current millisecond count
}
void loop()
{
 DateTime now = SoftRTC.now();
 long nowMS = millis();
 
 Serial.print(now.year(), DEC);
 Serial.print('/');
 Serial.print(now.month(), DEC);
 Serial.print('/');
 Serial.print(now.day(), DEC);
 Serial.print(' ');
 Serial.print(now.hour(), DEC);
 Serial.print(':');
 Serial.print(now.minute(), DEC);
 Serial.print(':');
 Serial.print(now.second(), DEC);
 Serial.print(':');
 Serial.print((nowMS - startMS)%1000, DEC); // print milliseconds
 Serial.println();

i tried to put this code that is based on ISR but with no luck

volatile time_t isrUTC; // ISR's copy of current time in UTC
volatile time_t millisecondsFromISR; // ISR's copy of current time millisecond offset
void incrementTime()
{
 millisecondsFromISR = millis(); //do first to get most accuracy
 ++isrUTC;
}
int getMilliseconds() {
 // need to make an atomic copy:
 nointerrupts();
 unsigned long t = millisecondsFromISR;
 interrupts();
 return millis() - t;
}

• jsotola: what is not accurate +/-2 milliseconds? ... the prints the time hh:mm:ss:ms takes time to execute
• Amr Ahmed: so if i make it with external clock will it make a diff?
• Edgar Bonet: You cannot get better than ±2 ms with millis(). If you need more accuracy, use micros().
• Amr Ahmed: same code or what do u suggest?

Answer 1

All you have to do is record the number of milliseconds at the moment the square wave toggles on its 1 second cycle. Then your milliseconds is the difference between that and the current.

In short: update startMS when triggered by the RTC's output.

• Amr Ahmed: do u have a helping code?
• Majenko: @AmrAhmed no, but you already have code to set startms in your program, and there are plenty of interrupt examples on the web.
• Amr Ahmed: yes i have checked these codes for interrupt with no luck to make a running code
• Majenko: @AmrAhmed Then maybe you should try again, and then show us what you tried, and we may be able to help you with it.
• Amr Ahmed: volatile time_t isrUTC; // ISR's copy of current time in UTC volatile time_t millisecondsFromISR; // ISR's copy of current time millisecond offset ... void incrementTime() { millisecondsFromISR = millis(); //do first to get most accuracy ++isrUTC; } ... int getMilliseconds() { // need to make an atomic copy: nointerrupts(); unsigned long t = millisecondsFromISR; interrupts(); return millis() - t; } this one of the codes that i tried to work on with no luck