AN4631 datasheet(10/30 Pages) STMICROELECTRONICS

10 / 30 page

Internal RC oscillator calibration

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DocID027266 Rev 2

Figure 4. TRIM monotonicity

2.1

Calibration principle

The calibration principle consists in:

Setting the internal RC oscillator (that needs to be calibrated) as system clock,

Measuring the internal RC oscillator (HSI16 or MSI) frequency for each trimming value,

Computing the frequency error for each trimming value,

Finally, setting the trimming bits with the optimum value (corresponding to the lowest

frequency error).

The internal oscillator frequency is not measured directly but is computed from the number

of clock pulses counted using a timer compared with the typical value. To do this, a very

accurate reference frequency must be available such as the LSE frequency provided by the

external 32.768 kHz crystal or the 50 Hz/60 Hz of the mains (refer to Section 2.2.2: Case

where another source is used as the reference frequency).

The following figure shows how the reference signal period is measured in number of timer

counts.

AN4631 Datasheet(PDF) 10 Page - STMicroelectronics