Clocks

AN4080

18/35

DocID023035 Rev 4

2.2

LSE clock

The LSE crystal is a 32.768 kHz Low Speed External crystal or ceramic resonator. It has the

advantage of providing a low-power but highly accurate clock source to the real-time clock

peripheral (RTC) for clock/calendar or other timing functions.

The LSE crystal is switched on and off using the LSEON bit in RTC domain control register

(RCC_BDCR). The crystal oscillator driving strength can be changed at runtime using the

LSEDRV[1:0] bits in the RTC domain control register (RCC_BDCR) to obtain the best

compromise between robustness and short start-up time on one side and low power-

consumption on the other.

The LSERDY flag in the RTC domain control register (RCC_BDCR) indicates whether the

LSE crystal is stable or not. At startup, the LSE crystal output clock signal is not released

until this bit is set by hardware. An interrupt can be generated if enabled in the Clock

interrupt register (RCC_CIR).

External source (LSE bypass)

In this mode, an external clock source must be provided. It can have a frequency of up to

1 MHz. You select this mode by setting the LSEBYP and LSEON bits in the RTC domain

control register (RCC_BDCR). The external clock signal (square, sinus or triangle) has to

drive the OSC32_IN pin while the OSC32_OUT pin can be used as GPIO. See Figure 7.

2.3

HSI 8 MHz clock

The HSI clock signal is generated from an internal 8 MHz RC oscillator and can be used

directly as a system clock or divided by 2 to be used as PLL input. The HSI RC oscillator

has the advantage of providing a clock source at low cost (no external components). It also

has a faster startup time than the HSE crystal oscillator however, even with calibration, the

frequency is less accurate than an external crystal oscillator or ceramic resonator.

Calibration

RC oscillator frequencies can vary from one chip to another due to manufacturing process

Furthermore, it is possible to route the HSI clock to the MCO multiplexer. The clock can then

be input to Timer 14 to allow the user to calibrate the oscillator.

2.4

LSI clock

The LSI RC acts as an low-power clock source that can be kept running in Stop and

Standby mode for the independent watchdog (IWDG) and RTC. The clock frequency is

around 40 kHz (between 30 kHz and 60 kHz). For more details, refer to the electrical

characteristics section of the datasheets.