Clocks

AN3216

18/33

DocID17496 Rev 7

3.3

LSE OSC clock

The low-speed external clock signal (LSE) can be generated from two possible clock

sources:

•

LSE user external clock (see Figure 10)

•

LSE external crystal/ceramic resonator (see Figure 11)

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.

2. To avoid exceeding the maximum value of CL1 and CL2 (15 pF), it is strongly recommended to use a

resonator with a load capacitance CL

3. The value of REXT depends on the crystal characteristics. A 0 W resistor works but, is not optimal. A

typical value is in the range of 5 to 6 RS (resonator series resistance). To fine tune the RS value refer to

AN2867 (Oscillator design guide for ST microcontrollers).

3.3.1

External source (LSE bypass)

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

32.768 kHz. The external clock signal (square, sine or triangle) with a duty cycle of about

50% has to drive the OSC32_IN pin while the OSC32_OUT pin must be left high impedance

(see Figure 10).

3.3.2

External crystal/ceramic resonator (LSE crystal)

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 oscillator can be switched on and off by software (default is off). When switched on, the

oscillator is not stable immediately. A bit is set in the RCC_CSR register when the oscillator

becomes stable and an interrupt can be generated if enabled in the RCC_CIR register.

The resonator and the load capacitors have to be connected as close as possible to the

oscillator pins in order to minimize output distortion and startup stabilization time. The load

capacitance values must be adjusted according to the selected oscillator (see Figure 11).

OSC32_OUT

OSC32_IN

External source

(Hi-Z)

ai14371

Hardware configuration

OSC32_OUT

OSC32_IN

ai14372d

STM32L1xxx

Hardware configuration

(3)