AN4467 datasheet(20/33 Pages) STMICROELECTRONICS

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Clocks

AN4467

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DocID026156 Rev 2

2.3

LSE OSC 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.

2.3.1

External crystal/ceramic resonator (LSE crystal)

The LSE crystal is switched on and off using the LSEON bit in RCC control/status register

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

LSEDRV[1:0] bits (in RCC_CSR register) to obtain the best compromise between

robustness and short start-up time on one side and low-power consumption on the other

(see Figure 10).

The LSERDY flag (in RCC_CSR) 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 enable register

(RCC_CIER).

2.3.2

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 RCC_CSR). The

external clock signal (square, sinus or triangle) has to drive the OSC32_IN pin, from current

consumption standpoint a square signal is preferred. The OSC32_OUT pin can be used as

GPIO (see Figure 10)

1. OSC32_IN and OSC_OUT pins can be also used as GPIOs, but it is recommended not to use them as

both RTC and GPIO pins in the same application.

Figure 10. External clock

Figure 11. Crystal/ceramic resonators

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