Reset and clock control (RCC) RM0367
186/1043 RM0367 Rev 7
The primary purpose of connecting the LSE to the channel 1 input capture is to be able to
accurately measure the HSI16 and MSI system clocks (for this, either the HSI16 or MSI
should be used as the system clock source). The number of HSI16 (MSI, respectively) 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 MSI and HSI16 oscillators both have dedicated user-accessible calibration bits for this
purpose.
The basic concept consists in providing a relative measurement (e.g. the HSI16/LSE ratio):
the precision is therefore closely related to the ratio between the two clock sources. The
higher the ratio, the better the measurement.
It is however not possible to have a good enough resolution when the MSI clock is low
(typically below 1 MHz). In this case, it is advised to:
• accumulate the results of several captures in a row
• use the timer’s input capture prescaler (up to 1 capture every 8 periods)
• use the RTC_OUT signal at 512 Hz (when the RTC is clocked by the LSE) as the input
for the channel1 input capture. This improves the measurement precision
TIM21 can also be used to measure the LSI, MSI, or HSE_RTC: this is useful for
applications with no crystal. The ultra-low-power LSI oscillator has a wide manufacturing
process deviation: by measuring it as a function of the HSI16 clock source, its frequency
can be determined with the precision of the HSI16.The HSE_RTC frequency (HSE divided
by a programmable prescaler) being relatively high (4 MHz), the relative frequency
measurement is not very accurate. Its main purpose is consequently to obtain a rough
indication of the external crystal frequency. This can be useful to meet the requirements of
the IEC 60730/IEC 61335 standards, which require to be able to determine harmonic or
subharmonic frequencies (–50/+100% deviations).
7.2.16 Clock-independent system clock sources for TIM2/TIM21/TIM22
In a number of applications using the 32.768 kHz clock as RTC timebase, timebases
completely independently from the system clock are useful. This allows to schedule tasks
without having to take into account the processor state (the processor may be stopped or
executing at low, medium or full speed).
For this purpose, the LSE clock is internally redirected to the 3 timers’ ETR inputs, which are
used as additional clock sources. This gives up to three independent time bases (using the
auto-reload feature) with 1 or 2 compare additional channels for fractional events. For
instance, the TIM21 auto-reload interrupt can be programmed for a 1 second tick interrupt
with an additional interrupt occurring 250 ms after the main tick.
Note: In this configuration, make sure that you have at least a ratio of 2 between the external clock
(LSE) and the APB clock. If the application uses an APB clock frequency lower than twice
the LSE clock frequency (typically LSE = 32.768 kHz, so twice LSE = 65.536 kHz), it is
mandatory to use the external trigger prescaler feature of the timer: it can divide the ETR
clock by up to 8.
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