AN2822

High speed internal oscillator trimming

Doc ID 14983 Rev 2

9/19

1.4

Measurement of HSI pulses per calibration period

known number of calibration signal periods (L), using the following equation:

TIM3 timer counts the number of HSI pulses with minimum interaction from the software

routine. This helps to reduce measurement uncertainty caused by the software routine

reaction times. In addition, the MCU core is not occupied by the software and can continue

to execute user code.

The measurement of HSI pulses starts with the initialization of TIM3 timer. It is configured to

count the number of HSI pulses and to handle two interrupt events. The first interrupt occurs

when the timer overflows. The second interrupt is an input capture interrupt which occurs

When the first rising edge of the calibration signal period occurs, the value in the

TIM3_CNTR register is stored in the input capture register, TIM3_CCR, and the content of

this register is saved during the input capture interrupt into the firstcapture variable.

detected and the value in the input capture register TIM3_CCR is saved into the lastcapture

variable.

The number of overflows between the first and last capture events is increased at every

overflow interrupt and stored in the overflow variable.

When the last input capture event occurs the state MEASURE_COMPLETED is set and the

timer is stopped. Then the number of HSI pulses is evaluated using the equation below:

Figure 4 summarizes the measurement of HSI pulses for a known calibration signal period.

f

HSI

N

HSI

T

cal

L

⋅

---------------------

=

N

HSI

lastcapture

0x10000

overflows

firstcapture

–

×

+

=