System Clock (Sysclk) Selection; Clock Security System (Css) - ST STM32F101 series Reference Manual

Low-, medium-, high- and XL-density reset and clock control (RCC)
LSI calibration
The frequency dispersion of the Low Speed Internal RC (LSI) oscillator can be calibrated to
have accurate RTC time base and/or IWDG timeout (when LSI is used as clock source for
these peripherals) with an acceptable accuracy.
This calibration is performed by measuring the LSI clock frequency with respect to TIM5
input clock (TIM5CLK). According to this measurement done at the precision of the HSE
oscillator, the software can adjust the programmable 20-bit prescaler of the RTC to get an
accurate time base or can compute accurate IWDG timeout.
Use the following procedure to calibrate the LSI:
1. Enable TIM5 timer and configure channel4 in input capture mode
2. Set the TIM5CH4_IREMAP bit in the AFIO_MAPR register to connect the LSI clock
internally to TIM5 channel4 input capture for calibration purpose.
3. Measure the frequency of LSI clock using the TIM5 Capture/compare 4 event or
interrupt.
4. Use the measured LSI frequency to update the 20-bit prescaler of the RTC depending
on the desired time base and/or to compute the IWDG timeout.
7.2.6

System clock (SYSCLK) selection

After a system reset, the HSI oscillator is selected as system clock. When a clock source is
used directly or through the PLL as system clock, it is not possible to stop it.
A switch from one clock source to another occurs only if the target clock source is ready
(clock stable after startup delay or PLL locked). If a clock source which is not yet ready is
selected, the switch will occur when the clock source will be ready. Status bits in the
control register (RCC_CR)
used as system clock.
7.2.7

Clock security system (CSS)

Clock Security System can be activated by software. In this case, the clock detector is
enabled after the HSE oscillator startup delay, and disabled when this oscillator is stopped.
If a failure is detected on the HSE clock, the HSE oscillator is automatically disabled, a clock
failure event is sent to the break input of the advanced-control timers (TIM1 and TIM8) and
an interrupt is generated to inform the software about the failure (Clock Security System
Interrupt CSSI), allowing the MCU to perform rescue operations. The CSSI is linked to the
®
Cortex -M3 NMI (Non-Maskable Interrupt) exception vector.
Note: Once the CSS is enabled and if the HSE clock fails, the CSS interrupt occurs and an NMI is
automatically generated. The NMI will be executed indefinitely unless the CSS interrupt
pending bit is cleared. As a consequence, in the NMI ISR user must clear the CSS interrupt
by setting the CSSC bit in the
If the HSE oscillator is used directly or indirectly as the system clock (indirectly means: it is
used as PLL input clock, and the PLL clock is used as system clock), a detected failure
causes a switch of the system clock to the HSI oscillator and the disabling of the HSE
oscillator. If the HSE clock (divided or not) is the clock entry of the PLL used as system clock
when the failure occurs, the PLL is disabled too.
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indicate which clock(s) is (are) ready and which clock is currently
Clock interrupt register
DocID13902 Rev 15
(RCC_CIR).
RM0008
Clock