Initially the internal calibrated RC oscillator was considered to be the main clock source for the circuit but according with the datasheet the frequency variation with the temperature is too high, specially on the operation range foreseen for the device which will be installed on the engine bay.

With the internal oscillator is out of game the solution is to use a crystal. The frequency chosen for the crystal is 16.0MHz because it is the same used in Arduino thus allowing me to prototype the circuit as a shield.