STM32F102XB datasheet(42/76 Pages) STMICROELECTRONICS

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Electrical characteristics

STM32F102x8, STM32F102xB

42/76

Doc ID 15056 Rev 4

Figure 18.

Typical application with an 8 MHz crystal

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 22. 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).

ai14977b

OSC_OU T

OSC_IN

fHSE

CL1

STM32F102xx

8 MH z

resonator

Bias

controlled

gain

REXT(1)

CL2

Resonator with

integrated capacitors

Table 22.

LSE oscillator characteristics (fLSE = 32.768 kHz)

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

C(1)

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)

RS = 30 kΩ

LSE driving current

VDD = 3.3 V

VIN = VSS

1.4

µA

Oscillator transconductance

µA/V

tSU(LSE)

(2)

Startup time

VDD is stabilized

TA = 50 °C

1.5

TA = 25 °C

2.5

TA = 10 °C

4.0

TA = 0 °C

6.0

TA = −10 °C

10.0

TA = −20 °C

17.0

TA = −30 °C

32.0

TA = −40 °C

60.0

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

ST microcontrollers”.

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 can vary significantly with the crystal manufacturer, PCB layout

and humidity.

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