i2c hang with RTC DS1307

Projects Networking, Protocols, and Devices
1

Hello all,
I've got a problem I can't understand so would appreciate any pointers people could give me!

I have an RTC (DS1307) connected via I2C to a mega 2560. Everything was working yesterday. This morning the board wouldn't run it's program and after some checking I found it had hung at the point of receiving the I2C data (via Wire library).

I copied the minimal code out to a simpler sketch where it works fine (see below). I have marked with comments where the same code hangs in the larger more complex sketch.

I can fix the hang by inserting a serial.print( "anything" ) just before the wire.available().

Any ideas why this would hang on the large sketch (and was working yesterday) but today has a problem, and works in a smaller sketch??

Thanks for any pointers!

#include <Wire.h>
#define DS1307_I2C_ADDRESS 0x68     // I2C bus address of DS1307 RTC

void setup()
{
  Wire.begin();        // join i2c bus (address optional for master)
  pinMode( 13, OUTPUT );
  Serial.begin(115200);  // start serial for output
  Serial.println( "started" );
  delay( 100 );

  Wire.beginTransmission(DS1307_I2C_ADDRESS);
  Wire.send(0);
  byte status = Wire.endTransmission();

  digitalWrite( 13, HIGH );
  Serial.println( "request 7 bytes" );
  Wire.requestFrom(DS1307_I2C_ADDRESS, 7);
  digitalWrite( 13, LOW );
 // HANGS between here
  while( Wire.available() < 7 )
  {
// AND here
    digitalWrite( 13, HIGH );
    delay( 10 );
    digitalWrite( 13, LOW );
    delay( 10 );
  }
  int sec = bcdToDec(Wire.receive()& 0x7f);    // 28/4: I've declared rtc variables globally so that log to eeprom could use them
  int minute = bcdToDec(Wire.receive());
  int hour = bcdToDec(Wire.receive()& 0x3f);
  int day = bcdToDec(Wire.receive());
  int date = bcdToDec(Wire.receive());
  int month = bcdToDec(Wire.receive());
  int year = bcdToDec(Wire.receive());
  //  log_debug( "got date bytes" );
  Serial.print( year );
}

byte bcdToDec(byte val)
{
  return ( (val/16*10) + (val%16) );
}

void loop()
{
}
2

I wouldn't be doing this personally:

  byte status = Wire.endTransmission();
  digitalWrite( 13, HIGH );
  Serial.println( "request 7 bytes" );
  Wire.requestFrom(DS1307_I2C_ADDRESS, 7);

After the endTransmission the slave is trying to respond, but you have wandered off and starting doing serial comms. Get rid of the Serial.println.

And this:

 while( Wire.available() < 7 )
...

So what happens if Wire.available() returns 6? You just loop forever.

Wire.requestFrom does a blocking read. Once you have exited that line you have all the data you are going to get. Going into a loop hoping Write.available() will go up just won't work.