Internal/External Clock Measurement Using Tim5/Tim11; Figure 16. Frequency Measurement With Tim5 In Input Capture Mode - ST STM32F446 Series Reference Manual

RM0390
The selected clock to output onto MCO must not exceed 100 MHz (the maximum I/O
speed).
6.2.11

Internal/external clock measurement using TIM5/TIM11

It is possible to indirectly measure the frequencies of all on-board clock source generators
by means of the input capture of TIM5 channel4 and TIM11 channel1 as shown in
and Figure
Internal/external clock measurement using TIM5 channel4
TIM5 has an input multiplexer which allows choosing whether the input capture is triggered
by the I/O or by an internal clock. This selection is performed through the TI4_RMP [1:0] bits
in the TIM5_OR register.
The primary purpose of having the LSE connected to the channel4 input capture is to be
able to precisely measure the HSI (this requires to have the HSI used as the system clock
source). The number of HSI clock counts between consecutive edges of the LSE signal
provides a measurement of the internal clock period. Taking advantage of the high precision
of LSE crystals (typically a few tens of ppm) we can 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. HSI/LSE ratio): the
precision is therefore tightly linked to the ratio between the two clock sources. The greater
the ratio, the better the measurement.
It is also possible to measure the LSI frequency: this is useful for applications that do not
have a crystal. The ultralow-power LSI oscillator has a large manufacturing process
deviation: by measuring it versus the HSI clock source, it is possible to determine its
frequency with the precision of the HSI. The measured value can be used to have more
accurate RTC time base timeouts (when LSI is used as the RTC clock source) and/or an
IWDG timeout with an acceptable accuracy.
Use the following procedure to measure the LSI frequency:
1. Enable the TIM5 timer and configure channel4 in Input capture mode.
2. This bit is set the TI4_RMP bits in the TIM5_OR register to 0x01 to connect the LSI
clock internally to TIM5 channel4 input capture for calibration purposes.
3. Measure the LSI clock frequency using the TIM5 capture/compare 4 event or interrupt.
4. Use the measured LSI frequency to update the prescaler of the RTC depending on the
desired time base and/or to compute the IWDG timeout.
17.

Figure 16. Frequency measurement with TIM5 in Input capture mode

RM0390 Rev 4
Reset and clock control (RCC)
Figure 16
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