STM32F107VC datasheet(50/108 Pages) STMICROELECTRONICS

50 / 108 page

Electrical characteristics

STM32F105xx, STM32F107xx

50/108

DocID15274 Rev 10

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 16). 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 the application note AN2867 “Oscillator design guide for ST

microcontrollers” available from the ST website www.st.com.

Figure 16. 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).

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

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

C(2)

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)(3)

RS = 30 kΩ

LSE driving current

VDD = 3.3 V, VIN = VSS

1.4

µA

Oscillator Transconductance

µA/V

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