STM32L151X8 datasheet(73/133 Pages) STMICROELECTRONICS

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STM32L151x6/8/B STM32L152x6/8/B

Electrical characteristics

104

Figure 17. HSE oscillator circuit diagram

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 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)

1. Guaranteed by characterization results.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

fLSE

Low speed external oscillator

frequency

32.768

kHz

Feedback resistor

1.2

C(2)

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)

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) Startup time

VDD is stabilized

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