STM32F102X8 datasheet(44/80 Pages) STMICROELECTRONICS

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Electrical characteristics

STM32F102x8, STM32F102xB

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DocID15056 Rev 5

High-speed external clock generated from a crystal/ceramic resonator

The high-speed external (HSE) clock can be supplied with a 4 to 16 MHz 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).

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 18). 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.

Table 21. HSE 4-16 MHz oscillator characteristics(1)(2)

1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.

2. Based on characterization results, not tested in production.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

fOSC_IN Oscillator frequency

MHz

Feedback resistor

200

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)(3)

3. The relatively low value of the RF resistor offers a good protection against issues resulting from use in a

humid environment, due to the induced leakage and the bias condition change. However, it is

recommended to take this point into account if the MCU is used in tough humidity conditions.

RS = 30 Ω

HSE driving current

VDD = 3.3 V

VIN = VSS with 30 pF load

1mA

Oscillator transconductance

Startup

mA/V

tSU(HSE)

(4)

4. tSU(HSE) is the startup time measured from the moment it is enabled (by software) to a stabilized 8 MHz

oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly

with the crystal manufacturer

Startup time

VDD is stabilized

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