STM32F427AI datasheet(121/240 Pages) STMICROELECTRONICS

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DS9405 Rev 12

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STM32F427xx STM32F429xx

Electrical characteristics

197

For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors in the

5 pF to 25 pF range (typ.), designed for high-frequency applications, and selected to match

the requirements of the crystal or resonator (see Figure 29). CL1 and CL2 are usually the

same size. The crystal manufacturer typically specifies a load capacitance, which is the

series combination of CL1 and CL2. PCB and MCU pin capacitance must be included (10 pF

can be used as a rough estimate of the combined pin and board capacitance) when sizing

CL1 and CL2.

Note:

For information on selecting the crystal, refer to the application note AN2867 “Oscillator

design guide for ST microcontrollers” available from the ST website www.st.com.

Figure 29. Typical application with an 8 MHz crystal

1. REXT value depends on the crystal characteristics.

Low-speed external clock generated from a crystal/ceramic resonator

The low-speed external (LSE) clock can be supplied with a 32.768 kHz crystal/ceramic

resonator oscillator. All the information given in this paragraph are based on

characterization results obtained with typical external components specified in Table 40. In

the application, the resonator and the load capacitors have to be placed as close as

possible to the oscillator pins to minimize output distortion and startup stabilization time.

Refer to the crystal resonator manufacturer for more details on the resonator characteristics

(frequency, package, accuracy).

Note:

For information on selecting the crystal, refer to the application note AN2867 “Oscillator

design guide for ST microcontrollers” available from the ST website www.st.com.

Table 40. LSE oscillator characteristics (fLSE = 32.768 kHz) (1)

1. Specified by design.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

18.4

MΩ

IDD

LSE current consumption

µA

ACCLSE(2)

2. This parameter depends on the crystal used in the application. Refer to application note AN2867.

LSE accuracy

− 500

500

ppm

Gm_crit_max Maximum critical crystal gm

Startup

0.56

µA/V

tSU(LSE)(3)

3. tSU(LSE) is the startup time measured from the moment that it is enabled (by software) to a stabilized

32.768 kHz oscillation is reached. This value is based on characterization and not tested in production. It is

measured for a standard crystal resonator and it can vary significantly with the crystal manufacturer.

startup time

VDD is stabilized

ai17530

OSC_OU T

OSC_IN

fHSE

CL1

STM32F

8 MHz

resonator

Resonator with

integrated capacitors

Bias

controlled

gain

REXT(1)

CL2

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