STM32F407IG datasheet(86/167 Pages) STMICROELECTRONICS

86 / 167 page

Electrical characteristics

STM32F405xx, STM32F407xx

86/167

Doc ID 022152 Rev 2

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 29). 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.

Figure 29.

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 29. 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).

Table 28.

HSE 4-26 MHz oscillator characteristics(1) (2)

Resonator characteristics given by the crystal/ceramic resonator manufacturer.

Based on characterization, 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)

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

-15

HSE driving current

VDD = 3.3 V, VIN =VSS

with 30 pF load

Oscillator transconductance

Startup

mA/V

tSU(HSE

(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

ai17530

OSC_OU T

OSC_IN

fHSE

CL1

STM32F

8 MH z

resonator

Resonator with

integrated capacitors

Bias

controlled

gain

REXT

(1)

CL2

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