DS3231 loosing time

i using cheap ds3231 rtc.

reading ct current
using emon openenergymonitor
using sd card level shifter 

lost 75 seconds overnight.  
been searching , found 1 reference in 1 post said reading rtc effect it's timing.   post said reads once day.  i assume uses millis() , adjusts discrepancy when evaluating data.

i happy second day. or two....

my second problem have no display , if sd card fails, no joy, no notification, , pin13 keeps blinking writing.

code: [select]


// * analog sensors on analog ins 0, 1, , 2
// * i2c bus rtc on a4 , a5
// * sd card attached spi bus follows:
// ** mosi - pin 11
// ** miso - pin 12
// ** clk - pin 13
// ** cs - pin 10

// libraries
#include <sd.h>
#include <spi.h>
#include <emonlib.h>
energymonitor emon1;    // create instance
// rtc initialization  *********************************
#include "wire.h"
#define ds3231_i2c_address 0x68

byte dectobcd(byte val) // convert normal decimal numbers binary coded decimal
{
  return ( (val / 10 * 16) + (val % 10) );
}
// convert binary coded decimal normal decimal numbers
byte bcdtodec(byte val)
{
  return ( (val / 16 * 10) + (val % 16) );
}
//  end rtc initialization ******************************

// pin assignments

// a1 assigned emon
int led50 = 6;
int led100 = 7;
const int chipselect = 10;

// global variables
int amps = 0;
double irms ;
int datalog = 0;
int old_datalog = 0;
float irmsdisplay ;

// fixed variables

void setup()
{
  wire.begin();
  serial.begin(9600); 
  pinmode(10, output);
  pinmode(13, output);

  emon1.current(1, 30);             // 1 = pin a1. 30 = fixed, ct has internal resistor.

  serial.print("initializing sd card...");

  if (!sd.begin(chipselect)) {     // see if card present , can initialized:
    serial.println("card failed, or not present");
    digitalwrite(13, high);   // turn led on (high voltage level)
    delay(2000);
    return;     // don't more:

  }
  serial.println("card initialized.");
}
//  rtc stuff ********************************
void readds3231time(
  byte *second,
  byte *minute,
  byte *hour,
  byte *dayofweek,
  byte *dayofmonth,
  byte *month,
  byte *year)
{
  wire.begintransmission(ds3231_i2c_address);
  wire.write(0); // set ds3231 register pointer 00h
  wire.endtransmission();
  wire.requestfrom(ds3231_i2c_address, 7);
  // request 7 bytes of data ds3231 starting register 00h
  *second = bcdtodec(wire.read() & 0x7f);
  *minute = bcdtodec(wire.read());
  *hour = bcdtodec(wire.read() & 0x3f);
  *dayofweek = bcdtodec(wire.read());
  *dayofmonth = bcdtodec(wire.read());
  *month = bcdtodec(wire.read());
  *year = bcdtodec(wire.read());
}
void displaytime()
{
  byte second, minute, hour, dayofweek, dayofmonth, month, year;
  // retrieve data ds3231
  readds3231time(&second, &minute, &hour, &dayofweek, &dayofmonth, &month,
                 &year);
  // concatenate writing sd card
  string datastring = "";   // make string assembling data log:

  datastring += string(hour, dec );
  datastring += string(":");
  if (minute < 10) datastring += string(0);
  datastring += string(minute, dec);
  datastring += string(":");
  if (second < 10) datastring += string(0);
  datastring += string(second, dec);
  datastring += string(" " );
  datastring += string(dayofmonth, dec );
  datastring += string("/" );
  datastring += string(month, dec );
  datastring += string( "/");
  datastring += string( year, dec);
  datastring += string(", - " );
   //  add value ct sensor
  datastring += (irmsdisplay);
  datastring += " amps,";

  file datafile = sd.open("datalog.txt", file_write);

  // if file available, write it:
  if (datafile) {
    datafile.println(datastring);
    datafile.close();
    // print serial port too:
    serial.println(datastring);
  }

}



// end  rtc stuff ********************************

void loop() {

  // ***** emon
  //  double irms = emon1.calcirms(1480);  // calculate irms // declared in inilitalization
  irms = emon1.calcirms(1480);  // calculate irms only
  irmsdisplay = irms;
  if (irms <= .25) {                 // added 0 dropout
    irmsdisplay = 0;
  }


  // ******************************************************************************************

  //  following how pick time create flag on when write sd card.


  if (amps >= 50) {
    datalog = high;
    digitalwrite(led50, low);
  }
  if (amps <= 40) {
    datalog = low;
    digitalwrite(led50, high);
  }
  if (amps >= 100) {
    datalog = high;
    digitalwrite(led100, low);
  }
  if (amps <= 80) {
    datalog = low;
    digitalwrite(led100, high);
  }

  if (datalog != old_datalog)
    // data log routine
    //  final bits of code go here

    old_datalog = datalog;

  // ****** end emon *************************************************

  displaytime(); // display real-time clock data on serial monitor,
  delay(1000); // every second
  serial.println();


  //  end rtc =======================================================


}


you bought either fake ds3231, or out of spec reject.
the genuine ones keep time 1-2 seconds/month.

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