STM32F103XC datasheet(57/123 Pages) STMICROELECTRONICS

57 / 123 page

STM32F103xC, STM32F103xD, STM32F103xE

Electrical characteristics

Doc ID 14611 Rev 7

57/123

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 22.

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 24. 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 (see Figure 23).

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

Table 24.

LSE oscillator characteristics (fLSE = 32.768 kHz)

(1)

Based on characterization, not tested in production.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

Feedback resistor



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 k

LSE driving current

VDD = 3.3 V, VIN = VSS

1.4

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

ai14145

OSC_OU T

OSC_IN

fHSE

CL1

STM32F103xx

8 MH z

resonator

REXT(1)

CL2

Resonator with

integrated capacitors

Bias

controlled

gain

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