ST STM32L152-EVAL Application Note

Clocks AN3216
16/30 Doc ID 17496 Rev 5
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 9)
● LSE external crystal/ceramic resonator (see Figure 10)
3. The value of R
EXT
depends on the crystal characteristics. A 0 Ω resistor works but, is not optimal. A typical
value is in the range of 5 to 6 R
S
(resonator series resistance). To fine tune the R
S
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 9).
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 10).
Figure 9. External clock
(1)(2)
1. 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. 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.
Figure 10. Crystal/ceramic resonators
(2)
OSC32_OUTOSC32_IN
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ST STM32L152-EVAL Specifications

General

Brand	ST
Model	STM32L152-EVAL
Category	Motherboard
Language	English