What's the benefit of having an external 32 kHz crystal?

Question

What are the benefits of having an external 32 kHz crystal instead of using the internal RC oscillator?

Ole Morten · Accepted Answer

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.

What's the benefit of having an external 32 kHz crystal?

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