STM32F103CBU6TR datasheet(53/116 Pages) STMICROELECTRONICS

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DS5319 Rev 18

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STM32F103x8, STM32F103xB

Electrical characteristics

115

For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors in the

5 pF to 25 pF range (typ.), designed for high-frequency applications, and selected to match

the requirements of the crystal or resonator (see Figure 24). CL1 and CL2 are usually the

same size. The crystal manufacturer typically specifies a load capacitance which is the

series combination of CL1 and CL2. PCB and MCU pin capacitance must be included (10 pF

can be used as a rough estimate of the combined pin and board capacitance) when sizing

CL1 and CL2. Refer to AN2867 “Oscillator design guide for ST microcontrollers”, available

from the ST website www.st.com.

Figure 24. Typical application with an 8 MHz crystal

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 23. 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).

ai14145

OSC_OU T

OSC_IN

fHSE

CL1

STM32F103xx

8 MH z

resonator

REXT(1)

CL2

Resonator with

integrated capacitors

Bias

controlled

gain

Table 23. LSE oscillator characteristics (fLSE = 32.768 kHz)(1) (2)

Symbol

Parameter

Conditions

Min

Typ

Max Unit

Feedback resistor

MΩ

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)(3) Startup time

VDD is

stabilized

TA = 50 °C

1.5

TA = 25 °C

2.5

TA = 10 °C

TA = 0 °C

TA = -10 °C

TA = -20 °C

TA = -30 °C

TA = -40 °C

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