STM32U385CG datasheet(130/224 Pages) STMICROELECTRONICS

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Low-speed external clock generated from a crystal resonator

The low-speed external (LSE) clock can be supplied with a 32.768 kHz crystal resonator oscillator. All the

information given in this paragraph are based on design simulation results obtained with typical external

components specified in the table below. 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).

Table 74. LSE oscillator characteristics (fLSE = 32.768 kHz)

Specified by design and not tested in production.

Symbol

Parameter

Conditions(1)

Min

Typ

Max

Unit

IDD(LSE)

LSE current consumption

LSEDRV[1:0] = 00, low-drive capability

340

LSEDRV[1:0] = 01, medium low-drive capability

380

LSEDRV[1:0] = 10, medium high-drive capability

520

LSEDRV[1:0] = 11, high-drive capability

660

Gmcritmax Maximum critical crystal Gm

LSEDRV[1:0] = 00, low-drive capability

0.5

µA/V

LSEDRV[1:0] = 01, medium low-drive capability

0.75

LSEDRV[1:0] = 10, medium high-drive capability

1.7

LSEDRV[1:0] = 11, high-drive capability

2.7

CS_PARA

Internal stray parasitic capacitance(2)

tSU(LSE)(3) Startup time

VDD is stabilized

1. Refer to the note below this table.

2. CS_PARA is the equivalent capacitance seen by the crystal due to OSC32_IN and OSC32_OUT internal parasitic capacitances.

3. 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 and it can vary significantly with the crystal manufacturer

Note:

For information on selecting the crystal, refer to the application note 'Oscillator design guide for STM8AF/AL/S,

STM32 MCUs and MPUs' (AN2867).

Figure 33. Typical application with a 32.768 kHz crystal

OSC32_IN

OSC32_OUT

Drive

programmable

amplifier

fLSE

32.768 kHz resonator

Resonator with integrated capacitors

CL1

CL2

Note: CL1 and CL2 are external load capacitances. Cs (stray capacitance) is the sum of the device OSC32_IN/OSC32_OUT pins

equivalent parasitic capacitance (CS_PARA), and the PCB parasitic capacitance.

Note:

An external resistor is not required between OSC32_IN and OSC32_OUT and it is forbidden to add one.

5.3.9

Internal clock timing characteristics

The parameters given in the tables below are derived from tests performed under ambient temperature and

supply voltage conditions summarized in Table 27. The curves provided are characterization results, not tested in

production.

STM32U385xx

Electrical characteristics

DS14830 - Rev 2

page 130/224

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