High speed internal oscillator calibration

AN3101

8/24

DocID16612 Rev 6

2.1

Calibration principle

The calibration principle consists in

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first measuring the HSI frequency for many calibration values, then

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computing the frequency error for each calibration value, and

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finally setting the HSITRIMR register with the optimum value.

The HSI frequency is not measured directly but it is estimated from the number of counted

HSI clock pulses using a timer compared with the typical value (16 MHz). To do so, 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).

Figure 3 shows how the reference signal period is measured in number of timer counts.

Figure 3. Timing diagram of HSI calibration

After enabling the timer counter, when the first rising edge occurs, the timer counter value is

registered in IC1ReadValue1. At the second rising edge, the timer counter is captured in

IC1ReadValue2. The elapsed time between two consecutive rising edges (IC1ReadValue2

and IC1ReadValue1) represents an entire period of the reference signal.

Since the timer is clocked by the internal HSI oscillator, the microcontroller can compute the

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The error (in Hz) is computed as the absolute value of the difference between the measured

After calculating the error for each calibration value, the algorithm determines the optimum

calibration value (the nearest value to 16 MHz) to be programmed in the HSITRIMR register

(refer to Section 2.3 for more details).

IC1ReadValue2 IC1ReadValue1

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Error Hz

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