AN2822

High speed internal oscillator trimming

Doc ID 14983 Rev 2

7/19

The HSI frequency is evaluated from the number of pulses counted by timer TIM3 during

certain calibration signal periods. For noisy mains, where positive and negative half-periods

are not necessarily symmetrical, it is better to measure the whole period and not only the

time difference between the rising and falling edge of the calibration signal.

It is better to measure the number of HSI pulses for several sequential periods to reduce the

uncertainty of the measurement. This is also recommended when using a noisy mains as

the calibration signal. The input capture prescaler can be used for this purpose. It allows

every 1st, 2nd, 4th, or 8th rising edge of the calibration signal to be selected which is then

used to define the time interval for counting HSI pulses. In this way TIM3 is able to store

information about the number of HSI pulses in the capture register at the beginning and at

the end of a measurement.

If required (for example, for validation purposes), the frequency of the HSI oscillator can be

observed on the configurable clock output (CCO) pin.

See Section 2 for an example of a simple post-processing circuit which adapts a mains

signal to a square signal and converts the mains voltage to 5 V power supply voltage.

1.3

Calibration routine

The calibration routine is written in ANSI C. It takes advantage of the STM8S and STM8A

firmware library which is available from www.st.com. It can be simplified and optimized if

needed.

The software compares the number of HSI clock pulses during the certain number L of

Using this equation, the calibration routine software compares the ideal HSI clock pulse

Note:

20 ms (if the mains has a frequency of 50 Hz), and the known number of calibration signal

periods, L, is 8 then the ideal value captured by the timer is 2 560 000 (0x271000)

The calibration algorithm is described in Figure 3.

Calibration starts with initialization of TIM3. The lowest frequency is set by writing

MAX_TRIM_VAL to the CLK_HSITRIMR register. In this way, the trimming value is

decreased in the program loop. The difference between the number of counted HSI pulses

and the ideal value is compared with the previous difference, until either the current

difference becomes greater than the preceding difference, or, until the minimum value

MIN_TRIM_VAL is reached.

If the new difference is greater than the preceding difference, the trimming value is

incremented to the previous value, resulting in the optimum value for calibration.

IdealCountedValue

f

HSI nominal

T

cal

×

L

×

=