DocID022913 Rev 2

9/27

AN4067

Internal RC oscillator calibration

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Figure 4. HSI48 oscillator trimming capability

3.1

Calibration principle

3.1.1

HSI calibration

The calibration principle consists in:

1.

Setting the HSI as the system clock source

2.

Measuring the internal RC oscillator (HSI) frequency for each trimming value

3.

Computing the frequency error for each trimming value

4.

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 external signal (refer to Section 3.2.1

and Section 3.2.2).

Figure 5 shows how the reference signal period is measured in number of timer counts.

Figure 5. Timing diagram of internal oscillator calibration