Measuring >40k interrupts per second by DS3231 32K and 8kHz enabled SQW lines on pins 2+3 of Arduino

gistfile1.txt

// needed for RTC.set(_, _), see bottom as well
#include <Wire.h>
#include <Time.h>
#include <DS1307RTC.h>
// from http://www.pjrc.com/teensy/td_libs_DS1307RTC.html


long lcnt=0;

void fcnt(void) { ++lcnt; }

void setup(void) {
    Serial.begin(57600);
    
/*
   enable 8kHz SQW square wave (is disabled on DSC3231 powerup)
   http://datasheets.maximintegrated.com/en/ds/DS3231.pdf#page=13
*/
    RTC.set(0x0E, 0x18);

    // do count interrupts from DS3231 pin 32K
    attachInterrupt(digitalPinToInterrupt(2), fcnt,  FALLING);
 
    // and count interrupts from DS3231 pin SQW
    attachInterrupt(digitalPinToInterrupt(3), fcnt,  FALLING);
}

void loop(void) {
    // output interrupts per second
    Serial.println(1000000.0*lcnt/micros());
    
    delay(500);
}


/*
added to DS1307RTC.h:
    static bool set(uint8_t reg, uint8_t val);

and to DS1307RTC.cpp:
bool DS1307RTC::set(uint8_t reg, uint8_t val)
{
  Wire.beginTransmission(DS1307_CTRL_ID);
  Wire.write(reg); // reset register pointer  
  Wire.write(val) ;   
  if (Wire.endTransmission() != 0) {
    return false;
  }
  return true;
}
*/

Photo showing wiring:
https://pbs.twimg.com/media/CSw0bFuWEAAza9D.jpg:large

from this tweet:
https://twitter.com/HermannSW/status/660960886597820416