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Clocks

AN4488

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4.2

LSE OSC clock

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

sources:

•

LSE user external clock (see Figure 20)

•

LSE external crystal/ceramic resonator (see Figure 21)

1. “LSE crystal/ceramic resonators” figure:

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 ≤7 pF. Never use a resonator with a load capacitance of 12.5 pF.

2. “LSE external clock” and “LSE crystal/ceramic resonators” figures:

OSC32_IN and OSC32_OUT pins can be used also as GPIO, but it is recommended not to use them as

both RTC and GPIO pins in the same application.

3. “LSE crystal/ceramic resonators” figure:

The value of REXT depends on the crystal characteristics. A 0 Ω resistor would work but would not be

optimal. To fine tube RS value, refer to AN2867 - Oscillator design guide for ST microcontrollers (Table 2).

4.2.1

External source (LSE bypass)

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

1 MHz. 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 20).

4.2.2

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 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.

Refer to the dedicated Application Note (AN2867 - Oscillator design guide for ST

microcontrollers) and electrical characteristics sections in the datasheet of your product for

more details.

Figure 20. LSE external clock

Figure 21. LSE crystal/ceramic

resonators

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