STM32L151VD datasheet(76/140 Pages) STMICROELECTRONICS

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Electrical characteristics

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Doc ID 022027 Rev 6

Figure 15.

HSE oscillator circuit diagram

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 29. 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 29.

LSE oscillator characteristics (fLSE = 32.768 kHz)

(1)

Based on characterization, not tested in production.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

fLSE

Low speed external oscillator

frequency

32.768

kHz

Feedback resistor

1.2

C(2)

Refer to the note and caution paragraphs below the table, and to the application note AN2867 “Oscillator

design guide for ST microcontrollers”.

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)

(3)

The oscillator selection can be optimized in terms of supply current using an high quality resonator with

small RS value for example MSIV-TIN32.768kHz. Refer to crystal manufacturer for more details.

RS = 30 kΩ

-8

ILSE

LSE driving current

VDD = 3.3 V, VIN = VSS

1.1

µA

IDD (LSE)

LSE oscillator current

consumption

VDD = 1.8 V

450

VDD = 3.0 V

600

VDD = 3.6V

750

Oscillator transconductance

µA/V

tSU(LSE)

(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 measured for a standard crystal resonator and it can vary

significantly with the crystal manufacturer.

Startup time

VDD is stabilized

OSC_OUT

OSC_IN

fHSE to core

CL1

CL2

STM32

Resonator

Consumption

control

Resonator

ai18235

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