STM32F102C4 datasheet(41/69 Pages) STMICROELECTRONICS

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STM32F102x4, STM32F102x6

Electrical characteristics

Doc ID 15057 Rev 3

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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).

Note:

For CL1 and CL2 it is recommended to use high-quality ceramic capacitors in the 5 pF to

15 pF range selected to match the requirements of the crystal or resonator. 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.

Load capacitance CL has the following formula: CL = CL1 x CL2 / (CL1 + CL2) + Cstray where

Cstray is the pin capacitance and board or trace PCB-related capacitance. Typically, it is

between 2 pF and 7 pF.

Table 22.

LSE oscillator characteristics (fLSE = 32.768 kHz)

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor



C(1)

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

design guide for ST microcontrollers”.

Recommended load capacitance

versus equivalent serial

resistance of the crystal (RS)

(2)

The oscillator selection can be optimized in terms of supply current using an high quality resonator with

small RS value for example MSIV-TIN32.768 kHz. Refer to crystal manufacturer for more details

RS = 30 k

LSE driving current

VDD = 3.3 V

VIN = VSS

1.4

µA

Oscillator transconductance

µA/V

tSU(LSE)

(3)

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

VDD is stabilized

ai14977b

OSC_OU T

OSC_IN

fHSE

CL1

STM32F102xx

8 MH z

resonator

Bias

controlled

gain

REXT(1)

CL2

Resonator with

integrated capacitors

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