Lse Osc Clock; External Source (Lse Bypass); External Crystal/Ceramic Resonator (Lse Crystal); Figure 20. Lse External Clock - ST STM32F4 Series Getting Started

Clocks
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
•
LSE external crystal/ceramic resonator (see

Figure 20. LSE external clock

1. "LSE crystal/ceramic resonators" figure:
To avoid exceeding the maximum value of C
resonator with a load capacitance C
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
optimal. To fine tube R
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.
28/44
Figure
≤
7 pF. Never use a resonator with a load capacitance of 12.5 pF.
L
depends on the crystal characteristics. A 0 Ω resistor would work but would not be
EXT
value, refer to AN2867 - Oscillator design guide for ST microcontrollers
S
DocID026304 Rev 3
20)
Figure 21)
Figure 21. LSE crystal/ceramic
and C (15 pF) it is strongly recommended to use a
L1 L2
AN4488
resonators
(Table 2).