ST STM32F20 Series User Manual

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:
To avoid exceeding the maximum value of C
L1
and C
L2
(15 pF) it is strongly recommended to use a
resonator with a load capacitance C
L
≤ 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 R
EXT
depends on the crystal characteristics. A 0 Ω resistor would work but would not be
optimal. To fine tube R
S
value, refer to AN2867 - Oscillator design guide for ST microcontrollers.
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_OUTOSC32_IN
External source
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ST STM32F20 Series Specifications

General

Brand	ST
Model	STM32F20 Series
Category	Microcontrollers
Language	English