External Crystal/Ceramic Resonator (Hse Crystal); Lse Osc Clock; Figure 18. Lse External Clock; Figure 19. Lse Crystal/Ceramic Resonators - ST STM32F7 Series Application Notes

AN4661
3.1.2

External crystal/ceramic resonator (HSE crystal)

The external oscillator frequency ranges from 4 to 26 MHz. The external oscillator has the
advantage of producing a very accurate rate on the main clock. The associated hardware
configuration is shown in
get accurate Ethernet, USB OTG high-speed peripheral, I2S and SAI.
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.
For CL1 and CL2 it is recommended to use high-quality ceramic capacitors in the 5 pF-to-
25 pF range (typ.), designed for high-frequency applications and selected to meet the
requirements of the crystal or resonator. CL1 and CL2, are usually the same value. The
crystal manufacturer typically specifies a load capacitance that is the series combination of
CL1 and CL2. The PCB and MCU pin capacitances must be included when sizing CL1 and
CL2 (10 pF can be used as a rough estimate of the combined pin and board capacitance).
The HSERDY flag in the RCC clock control register (RCC_CR) indicates if the high-speed
external oscillator is stable or not. At startup, the clock 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).
The HSE crystal can be switched on and off using the HSEON bit in the RCC clock control
register (RCC_CR).
3.2

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 18. LSE external clock

1. Figure 19: 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 18: 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).
Figure 17. Using a 25 MHz oscillator frequency is a good choice to
Figure 18).

Figure 19. LSE crystal/ceramic resonators

resonators:
and Figure 19: LSE crystal/ceramic
DocID027559 Rev 5
Figure 19).
resonators:
Clocks
25/54
53