STM32F479II datasheet(117/217 Pages) STMICROELECTRONICS

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STM32F479xx

Electrical characteristics

190

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 informations 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).

Note:

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

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

Figure 32. Typical application with a 32.768 kHz crystal

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(2)

2. LSE mode cannot be changed “on the fly” otherwise, a glitch can be generated on OSCIN pin.

µA

High drive mode(2)

ACCLSE(3)

3. 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

Low power mode(2)

0.56

µA/V

High drive mode(2)

1.5

tSU(LSE)(4)

4. 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 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

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