AVR Embedded Tutorial - I²C External-Clock

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I²C external clock DS3231

This code is for an Atmega328 / Arduino with 16MHz clock and a DS3231 clock module.

The following functions are described in other tutorials:

• Delays
• UART - serial interface
• Activate I²C bus or AVR Embedded Tutorial - Software I2C, TWI

This example shows how to address the DS3231 clock module, which is connected to the I²C bus.

PullUp resistors are necessary, the DS3231 does not contain any itself.

Functions for the DS3231 control

Constants

 const
   I2CAddr   = $68;  // Default address of the DS3231.
   TWI_Write = 0;
   TWI_Read  = 1;

Record for time storage

 type
   TDate = record
     year : uint16;
     month, day, hour, minute, second, dayOfTheWeek : uint8;
   end;
 var
   Date : TDate;

BCD to BIN, BIN to BCD

These conversion functions are required because the time is in BCD format.

  function bcd2bin(val : uint8) : uint8;
   begin
     Result := val - 6 * (val shr 4);
   end;

   function bin2bcd(val : uint8) : uint8;
   begin
     Result := val + 6 * (val div 10);
   end;

Write the time

Write the block with a time. You can see that you have to convert the format.

  procedure WriteDS3231(addr : UInt16);
   begin
     TWIStart((addr shl 1) or TWI_Write);
     TWIWrite(0);
     TWIWrite(bin2bcd(Date.Second));
     TWIWrite(bin2bcd(Date.Minute));
     TWIWrite(bin2bcd(Date.Hour));
     TWIWrite(bin2bcd(0));
     TWIWrite(bin2bcd(Date.Day));
     TWIWrite(bin2bcd(Date.Month));
     TWIWrite(bin2bcd(Date.Year - 2000));
     TWIStop;
   end;

Read the time

Read the time from the block. You can see that you have to convert the format.

  procedure ReadDS3231(addr : UInt16);
   begin
     TWIStart((addr shl 1) or TWI_Write);
     TWIWrite(0);
     TWIStop;

     TWIStart((addr shl 1) or TWI_Read);
     Date.second := bcd2bin(TWIReadACK_Error and $7F);
     Date.minute := bcd2bin(TWIReadACK_Error);
     Date.hour   := bcd2bin(TWIReadACK_Error);

     TWIReadACK_Error;

     Date.day    := bcd2bin(TWIReadACK_Error);
     Date.month  := bcd2bin(TWIReadACK_Error);
     Date.year   := bcd2bin(TWIReadNACK_Error) + 2000;

     TWIStop;
   end;

Example program

Variables

  var
   s : ShortString;

Main program

You only have to write the time to the DS3231 to if you want to set the clock.

  begin
   UARTInit;     // initialize UART
   TWIInit;      // initialize I²C
   asm Sei end;  // enable interrupts

   // Write a time from the DS3231.
   Date.second := 33;
   Date.minute := 22;
   Date.hour   := 11;

   WriteDS3231(I2CAddr);

   // Read time and output via UART
   repeat
     ReadDS3231(I2CAddr);

     str(Date.second : 6, s);
     UARTSendString('Sec:' + s);
     str(Date.minute : 6, s);
     UARTSendString('Minute:' + s);
     str(Date.hour : 6, s);
     UARTSendString('hours:' + s);

     UARTSendString(#13#10);
   until 1 = 2;
 end.

See also

• Overview page - AVR Embedded Tutorials.