STM32F215RG datasheet(83/173 Pages) STMICROELECTRONICS

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STM32F21xxx

Electrical characteristics

Doc ID 17050 Rev 8

83/173

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 30). 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.

Note:

For information on electing the crystal, refer to the application note AN2867 “Oscillator

design guide for ST microcontrollers” available from the ST website www.st.com.

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 30.

Typical application with a 32.768 kHz crystal

Table 28.

LSE oscillator characteristics (fLSE = 32.768 kHz)

(1)

Guaranteed by design, not tested in production.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor

18.4

IDD

LSE current consumption

µA

Oscillator Transconductance

2.8

µA/V

tSU(LSE)

(2)

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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