An external 32 kHz crystal will give lower average current consumption than using the RC oscillator, at the expense of cost and board space. There are primarily 3 reasons the current consumption is lower:
The RC oscillator must be calibrated every 4 seconds to be accurate enough. Calibration normally takes about 17 ms, and requires the 16 MHz to be running, giving an average current consumption increase of about 6-7 µA.
The receive window must be widened to account for the worse accuracy. Since the RC is 250 ppm, compared with typically 20-30 ppm for an external crystal, the radio must receive for a slightly longer time, giving an increase of 1-2 µA average
The run current or the RC is 0.8 µA instead of 0.4 µA for a typical crystal.
In total, this gives 8-10 µA extra average current consumption. Depending on application, this may be enough to justify the cost and board space for a crystal.
I'm not absolutely certain whether the calibration interval is the same for the nRF8001, but any needed calibration of the nRF8001's RC oscillator is anyway handled internally in the chip, and no external actions are needed to make it happen.
I'm not absolutely certain whether the calibration interval is the same for the nRF8001, but any needed calibration of the nRF8001's RC oscillator is anyway handled internally in the chip, and no external actions are needed to make it happen.
