Clocks

AN3320

14/29

Doc ID 18267 Rev 2

2.2

LSE OSC clock

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

sources:

●

LSE external crystal/ceramic resonator (see Figure 9)

●

LSE user external clock (see Figure 8)

1.

“LSE crystal/ceramic resonators” figure:

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:

2.2.1

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 8).

2.2.2

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.

Figure 8.

LSE external clock

Figure 9.

LSE crystal/ceramic

resonators

OSC32_OUT

OSC32_IN

External source

(Hi-Z)

ai14371

Hardware configuration