STM32F207IE datasheet(78/163 Pages) STMICROELECTRONICS

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Electrical characteristics

STM32F205xx, STM32F207xx

78/163

Doc ID 15818 Rev 7

Note:

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

5 pF to 15 pF range selected to match the requirements of the crystal or resonator (see

Figure 31). 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.

Caution:

To avoid exceeding the maximum value of CL1 and CL2 (15 pF) it is strongly recommended

to use a resonator with a load capacitance CL ≤ 7 pF. Never use a resonator with a load

capacitance of 12.5 pF.

Example: if you choose a resonator with a load capacitance of CL = 6 pF, and Cstray = 2 pF,

then CL1 = CL2 = 8 pF.

Figure 31.

Typical application with a 32.768 kHz crystal

Table 26.

LSE oscillator characteristics (fLSE = 32.768 kHz)

(1)

Based on characterization, not tested in production.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

18.4

C(2)

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)

(3)

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

3.5

µA

Oscillator Transconductance

µA/V

tSU(LSE)

(4)

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

ai17531

OSC32_OU T

OSC32_IN

fLSE

CL1

STM32F

32.768 kH z

resonator

Resonator with

integrated capacitors

Bias

controlled

gain

CL2

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