STM32F411CE datasheet(85/149 Pages) STMICROELECTRONICS

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STM32F411xC STM32F411xE

Electrical characteristics

123

Figure 24. 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 38. In

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

possible to the oscillator pins in order to minimize output distortion and startup stabilization

time. Refer to the crystal resonator manufacturer for more details on the resonator

characteristics (frequency, package, accuracy).

The LSE high-power mode allows to cover a wider range of possible crystals but with a cost

of higher power consumption.

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.

For information about the LSE high-power mode, refer to the reference manual RM0383.

Table 38. LSE oscillator characteristics (fLSE = 32.768 kHz) (1)

1. Guaranteed by design.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

18.4

IDD

LSE current

consumption

Low-power mode

(default)

µA

High-drive mode

Gm_crit_max

Maximum critical crystal

Startup, low-power mode

0.56

µA/V

Startup, high-drive mode

1.50

tSU(LSE)(2)

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

32.768 kHz oscillation is reached. This value is guaranteed by characterization. It is measured for a

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

startup time

VDD is stabilized

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