TWI Wire.setClock() on non arduino AVR MCU issue

Question

Using an Atmega168P - programmed by means of avrdude with the .hex file generated by Arduino IDE.

The following code sets the 8 MHz Atmega's TWI interface to just 15.625 kHz:

  Wire.begin(); // Initiate the Wire library
  Wire.setClock(100* 1000);
  delay(10);

While this one doubles it to 31.25 kHz:

  Wire.begin(); // Initiate the Wire library
  TWBR = (F_CPU/(100*1000) - 16)/2;
  TWSR &= 0b11111100;
  delay(10);

Looking to get 100 kHz clock. What am I missing ?

Calculations are int (16-bits) by default. Try 100*1000L. Or better define F_TWI as 100000L and use TWBR = ((F_CPU/F_TWI)-16)/2; — Mikael Patel
– Mikael Patel, Commented Apr 18, 2017 at 12:02
@MikaelPatel That was it! You may want to turn that into an answer as well, thanks. — kellogs
– kellogs, Commented Apr 18, 2017 at 12:10
You get 100kHz with just Wire.begin(), because that is the default. The Wire.setClock has limits (I don't know the limits) but 50kHz to 200kHz should work: Wire.setClock(50000L) to Wire.setClock(200000L). I hope you are not trying to compile 16MHz code for a 8MHz ATmega chip. If there is no 8MHz ATmega168P in the menu, you should create an entry in boards.txt. — Jot
– Jot, Commented Apr 18, 2017 at 13:46

Mikael Patel · Accepted Answer · 2017-04-18 12:57:50Z

3

Looking to get 100 kHz clock. What am I missing?

Calculations are int (16-bits) by default in AVR GCC. Try 100*1000L. Or even better:

#define F_TWI 100000L

and use:

TWBR = ((F_CPU/F_TWI)-16)/2;

or:

Wire.setClock(F_TWI);

Cheers!

answered Apr 18, 2017 at 12:57

Mikael Patel

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