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DS1307

There are two almost identical breakout boards available - one from SparkFun for $20 and one from Adafruit for $9. I prefer the one from Adafruit. It is lightly larger, but it is much cheaper, has a better pin layout, and it is better supported.

The Register

It is good to read the complete DS1307 Datasheet , but here is a summary:

Address 7 6 5 4 3 2 1 0 Function Range
0x00 CH 10 Seconds 1 Seconds Seconds 0x00-0x59
0x01 0 10 Minutes 1 Minutes Minutes 0x00-0x59
0x02 0 12h AM/PM 10 Hour 1 Hour Hours 0x01-0x12
24h 10 Hour 0x00-0x23
0x03 0 0 0 0 0 Day Day 0x01-0x07
0x04 0 0 10 Date 1 Date Date 0x01-0x31
0x05 0 0 0 10 Month 1 Month Month 0x01-0x12
0x06 10 Year 1 Year Year 0x00-0x99
0x07 OUT 0 0 SQWE 0 0 RS1 RS0 Control -
0x08-0x3F RAM RAM 0x00-0xFF

Bus Pirate

Setup

HiZ> m 4 3 1
I2C (mod spd)=( 0 2 )
Ready
I2C> W
Power supplies ON
I2C> P
Pull-up resistors ON
I2C>

Setting Clock

I2C> [ 0xd0 0 0x00 0x20 0x16 3 0x01 0x09 0x10 0 ]
I2C START BIT
WRITE: 0xD0 ACK
WRITE: 0x00 ACK
WRITE: 0x00 ACK
WRITE: 0x20 ACK
WRITE: 0x16 ACK
WRITE: 0x02 ACK
WRITE: 0x01 ACK
WRITE: 0x09 ACK
WRITE: 0x10 ACK
WRITE: 0x00 ACK
I2C STOP BIT
I2C>

   Wire.beginTransmission(DS1307_ADDRESS);
   Wire.send(0);
   Wire.send(bin2bcd(dt.second()));
   Wire.send(bin2bcd(dt.minute()));
   Wire.send(bin2bcd(dt.hour()));
   Wire.send(bin2bcd(0));
   Wire.send(bin2bcd(dt.day()));
   Wire.send(bin2bcd(dt.month()));
   Wire.send(bin2bcd(dt.year() - 2000));
   Wire.send(0);
   Wire.endTransmission();

Reading the Clock

I2C> [ 0xd0 0 ] [ 0xd1 r:7 ]
I2C START BIT
WRITE: 0xD0 ACK
WRITE: 0x00 ACK
I2C STOP BIT
I2C START BIT
WRITE: 0xD1 ACK
READ: 0x00  ACK 0x20  ACK 0x16  ACK 0x03  ACK 0x01  ACK 0x09  ACK 0x10
NACK
I2C STOP BIT
I2C>

  Wire.beginTransmission(DS1307_ADDRESS);
  Wire.send(0);
  Wire.endTransmission();

  Wire.requestFrom(DS1307_ADDRESS, 7);
  uint8_t ss = bcd2bin(Wire.receive() & 0x7F);
  uint8_t mm = bcd2bin(Wire.receive());
  uint8_t hh = bcd2bin(Wire.receive());
  Wire.receive();
  uint8_t d = bcd2bin(Wire.receive());
  uint8_t m = bcd2bin(Wire.receive());
  uint16_t y = bcd2bin(Wire.receive()) + 2000;

DS3231