External Clock (Lse Bypass); External Crystal/Ceramic Resonator (Lse Crystal); Figure 13. Lse External Clock; Figure 14. Lse Crystal/Ceramic Resonators - ST STM32H7 3 Series Application Note

AN4938

Figure 13. LSE external clock

1. Figure 14: LSE crystal/ceramic
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.
2. Figure 13: LSE external clock
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.
The LSE oscillator is switched ON and OFF using the LSEON bit in RCC backup domain
control register (RCC_BDCR).
The LSE oscillator includes new modes and has a configurable drive using the LSEDRV
[1:0] in RCC_BDCR register:
•
00: Low drive.
•
10: Medium low drive.
•
01: Medium high drive.
•
11: High drive.
The LSERDY flag in the RCC backup domain control register (RCC_BDCR) indicates if the
LSE crystal is stable or not. At startup, the LSE crystal output clock signal is not released
until this bit is set by hardware. An interrupt can be generated if enabled in the RCC clock
interrupt register (RCC_CIR).
3.2.1

External clock (LSE bypass)

The LSE bypass mode is available in all system power modes except for Standby and V
An external clock source must be provided in LSE bypass mode. It must have a frequency
up to 1 MHz. The user selects this mode by setting the LSEBYP and LSEON bits in the RCC
backup domain control register (RCC_BDCR). The external clock signal (square, sinus or
triangle) with ~50% duty cycle has to drive the OSC32_IN pin while the OSC32_OUT pin
should be left HI-Z. See
3.2.2

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 the output distortion and startup stabilization time. The
load capacitance values must be adjusted according to the selected oscillator.
resonators:
and Figure 14: LSE crystal/ceramic
Figure 13.
DocID029918 Rev 1

Figure 14. LSE crystal/ceramic resonators

resonators:
Clocks
.
BAT
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