Low speed internal oscillator measurement

AN3101

20/24

DocID16612 Rev 6

Figure 12. Timing diagram of LSI measurement

The LSI frequency value is computed as shown by the following formula:

LSI_Frequency = HSI_Value / Capture

where:

•

HSI_Value is the HSI frequency value (typically 16 MHz);

•

Capture represents an entire LSI period: IC1ReadValue2 - IC1ReadValue1.

As you can conclude from the formula above, the LSI measurement accuracy depends on

the HSI frequency accuracy. Consequently, if an external reference signal is available, the

user can run the HSI calibration routine described in Section 2 before performing the LSI

measurement procedure in order to get a more accurate HSI_Value.

The Input capture divider (or prescaler) can be used for better measurement accuracy. In

this case, the formula above becomes:

LSI_Frequency = InputCaptureDivider * HSI_Value / Capture_Value.

3.2

Description of the LSI clock measurement firmware

The LSI clock measurement firmware provided with this application note includes one major

function: LSI_FreqMeasure(void).

The LSI_FreqMeasure() function measures the LSI frequency value. It measures a

predefined number of LSI periods. Then it returns the average value to minimize the error of

measured frequency. The user can change this parameter (number of LSI periods) in the

lsi_measurement.h file.

#define LSI_PERIOD_NUMBERS 10

By default, the number of measured periods per frequency is set to 10.

To start the LSI measurement, save the user clock configuration and set the system

configuration for LSI measurement (select the HSI clock as system clock, configure Timer 2

channel 1 in input capture mode, connect LSI to Timer 2 channel 1...).

After system configuration, Timer 2 counter and capture 1 interrupt are enabled. This makes

it possible to save the captured values for the predefined number of LSI periods. Once the

measurement is performed, the average value is computed and returned after restoring the

system clock configuration. Figure 13 gives the flowchart of the LSI measurement.