STM32F103C8T6 datasheet(36/67 Pages) STMICROELECTRONICS

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Electrical characteristics

STM32F103xx

36/67

Low-speed external clock

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 18. 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).

Figure 14.

Typical application with a 32.768 kHz crystal

Table 18.

LSE oscillator characteristics (fLSE = 32.768 kHz)

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

Ω

CL1

CL2

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)

(1)

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

LSE driving current

VDD = 3.3 V

VIN = VSS

1.4

µA

Oscillator Transconductance

µA/V

tSU(LSE)

(2)

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

VSS is stabilized

ai14146

OSC32_OU T

OSC32_IN

fLSE

CL1

STM32F103xx

32.768 kH z

resonator

CL2

Resonator with

integrated capacitors

Bias

controlled

gain

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