Internal/External Clock Measurement With Tim16; Figure 16. Frequency Measurement With Tim16 In Capture Mode - STMicroelectronics RM0365 Reference Manual

Reset and clock control (RCC)
9.2.14

Internal/external clock measurement with TIM16

It is possible to indirectly measure the frequency of all on-board clock sources by mean of
the TIM16 channel 1 input capture. As represented on
The input capture channel of the Timer 16 can be a GPIO line or an internal clock of the
MCU. This selection is performed through the TI1_RMP [1:0] bits in the TIM16_OR register.
The possibilities available are the following ones.
• TIM16 Channel1 is connected to the GPIO. Refer to the alternate function mapping in
the device datasheets.
•
TIM16 Channel1 is connected to the RTCCLK.
•
TIM16 Channel1 is connected to the HSE/32 Clock.
•
TIM16 Channel1 is connected to the microcontroller clock output (MCO), this selection
is controlled by the MCO[2:0] bits of the Clock configuration register (RCC_CFGR).
Calibration of the HSI
The primary purpose of connecting the LSE, through the MCO multiplexer, to the channel 1
input capture is to be able to precisely measure the HSI system clocks (for this, the HSI
should be used as the system clock source). The number of HSI clock counts between
consecutive edges of the LSE signal provides a measure of the internal clock period. Taking
advantage of the high precision of LSE crystals (typically a few tens of ppm's), it is possible
to determine the internal clock frequency with the same resolution, and trim the source to
compensate for manufacturing-process- and/or temperature- and voltage-related frequency
deviations.
The HSI oscillator has dedicated user-accessible calibration bits for this purpose.
The basic concept consists in providing a relative measurement (e.g. the HSI/LSE ratio): the
precision is therefore closely related to the ratio between the two clock sources. The higher
the ratio is, the better the measurement will be.
If LSE is not available, HSE/32 will be the better option in order to reach the most precise
calibration possible.
Calibration of the LSI
The calibration of the LSI will follow the same pattern that for the HSI, but changing the
reference clock. It will be necessary to connect LSI clock to the channel 1 input capture of
the TIM16. Then define the HSE as system clock source, the number of his clock counts
between consecutive edges of the LSI signal provides a measure of the internal low speed
clock period.
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Figure 16. Frequency measurement with TIM16 in capture mode

DocID025202 Rev 7
Figure 16.
RM0365