DocID027559 Rev 1

17/33

AN4661

Clocks

32

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 Figure 9. Using a 25 MHz oscillator frequency is a good choice to

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 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 Figure 10).

•

LSE external crystal/ceramic resonator (see Figure 11).

1. Figure 11: LSE crystal/ceramic resonators:

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 10: LSE external clock and Figure 11: LSE crystal/ceramic resonators:

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 10. LSE external clock

Figure 11. LSE crystal/ceramic resonators