Internal RC oscillator calibration

AN4067

10/27

DocID022913 Rev 2

After enabling the timer counter, when the first rising edge of the reference signal occurs,

the timer counter value is captured and stored in IC1ReadValue1. At the second rising edge,

the timer counter is captured again and stored in IC1ReadValue2. The elapsed time

between two consecutive rising edges (IC1ReadValue2 - IC1ReadValue1) represents an

entire period of the reference signal.

Since the timer counter is clocked by the system clock (internal RC oscillator HSI), the real

frequency generated by the internal RC oscillator versus the reference signal is given by:

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

frequency and the typical value.

Hence, the internal oscillator frequency error is expressed as:

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

trimming value (that corresponds to the nearest frequency to typical value) to be

programmed in the trimming bits (refer to Section 3.3: Description of the internal oscillator

calibration firmware for more details).

3.1.2

HSI14 calibration

The calibration principle consists in:

1.

Setting the PLL, clocked by the HSI (after calibrating the HSI), as the system clock

source.

2.

Measuring the internal RC oscillator (HSI14) frequency for each trimming value.

3.

Computing the frequency error for each trimming value.

4.

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

frequency error).

Figure 6 shows how the reference signal period is measured in number of timer counts.

Figure 6. Timing diagram of internal oscillator calibration HSI14

After enabling the timer counter, when the first rising edge of the reference signal occurs,

the timer counter value is captured and stored in IC1ReadValue1. At the second rising edge,

the timer counter is captured again and stored in IC1ReadValue2. The elapsed time

between two consecutive rising edges (IC1ReadValue2 - IC1ReadValue1) represents an

entire period of the reference signal.

Measuredfrequency

IC1ReadValue2

IC1ReadValue1

–

 referencefrequency



=

Error Hz



Measuredfrequency

typicalvalue

–

=