STM32L072X8 datasheet(83/147 Pages) STMICROELECTRONICS

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STM32L072xx

Electrical characteristics

117

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 46. 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 the ST website www.st.com.

Figure 24. Typical application with a 32.768 kHz crystal

Note:

An external resistor is not required between OSC32_IN and OSC32_OUT and it is forbidden

to add one.

Table 46. LSE oscillator characteristics(1)

Symbol

Parameter

Conditions(2)

Min(2)

Typ

Max

Unit

fLSE

LSE oscillator frequency

32.768

kHz

Maximum critical crystal

transconductance

LSEDRV[1:0]=00

lower driving capability

0.5

µA/V

LSEDRV[1:0]= 01

medium low driving capability

0.75

LSEDRV[1:0] = 10

medium high driving capability

1.7

LSEDRV[1:0]=11

higher driving capability

2.7

tSU(LSE)(3) Startup time

VDD is stabilized

1. Guaranteed by design.

2. Refer to the note and caution paragraphs below the table, and to the application note AN2867 “Oscillator design guide for

ST microcontrollers”.

3. Guaranteed by characterization results. 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 measured for a standard crystal resonator and it can vary

significantly with the crystal manufacturer. To increase speed, address a lower-drive quartz with a high- driver mode.

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