DocID023944 Rev 4

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AN4206

Clocks

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External crystal/ceramic resonator (HSE crystal)

The 4 to 32 MHz external oscillator has the advantage of producing a very accurate rate on

the main clock. Refer to the electrical characteristics section of the datasheet for more

details about the associated hardware configuration.

The HSERDY flag in the Clock control register (RCC_CR) indicates if the HSE oscillator is

stable or not. At startup, the clock is not released until this bit is set by hardware. An

interrupt can be generated if enabled in the Clock interrupt register (RCC_CIR).

The HSE Crystal can be switched on and off using the HSEON bit in the Clock control

register (RCC_CR).

External source (HSE bypass)

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

cycle depending on the frequency (refer to the datasheet) has to drive the OSC_IN pin while

the OSC_OUT pin can be used a GPIO. See Figure 9.

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 Backup domain control

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

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

the best compromise between robustness and short start-up time on the one hand and low

power consumption on the other.

The LSERDY flag in the Backup 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 Backup domain

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

~50% duty cycle has to drive the OSC32_IN pin while the OSC32_OUT pin can be used as

GPIO. See Figure 9.

2.3

HSI 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.