DocID16612 Rev 6

19/24

AN3101

Low speed internal oscillator measurement

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Low speed internal oscillator measurement

The low-speed internal oscillator (LSI) is a low power clock source that can be kept running

in Active-halt and Run modes for the independent watchdog (IWDG). The LSI oscillator can

be switched on and off using the LSION bit in the internal clock register (CLK_ICKCR). The

LSI clock is an RC oscillator that can vary, due to environment temperature and supply

voltage, from 26 kHz to 56 kHz. This is the reason why a measurement of its real value is

required to avoid an unexpected behavior due to LSI frequency variation. In the

STM8L05xxx/15xxx, STM8L162xx and STM8AL31xx/3Lxx microcontrollers, an internal

connection between Timer 2 channel 1 and the LSI clock is provided to simplify the LSI

measurement procedure, as shown in Figure 10.

Figure 10. Internal connection between LSI and Timer 2 channel 1

3.1

LSI measurement principle

The low-speed internal oscillator (LSI) measurement procedure consists in running the

TIM2 counter using the HSI clock, configuring the Timer 2 channel 1 in input capture mode

and then connecting the LSI clock to Timer 2 channel 1. Figure 11 shows the configuration

used to perform the LSI measurement.

Figure 11. LSI measurement configuration

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

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

IC1ReadValue2. The elapsed time between two consecutive rising edges of the LSI clock

represents an entire period. Figure 12 shows the timing diagram of LSI measurement.