STM32F405 datasheet(101/204 Pages) STMICROELECTRONICS

101 / 204 page

DS8626 Rev 9

101/203

STM32F405xx, STM32F407xx

Electrical characteristics

Figure 31. Low-speed external clock source AC timing diagram

High-speed external clock generated from a crystal/ceramic resonator

The high-speed external (HSE) clock can be supplied with a 4 to 26 MHz crystal/ceramic

resonator oscillator. All the information given in this paragraph are based on

characterization results obtained with typical external components specified in Table 32. 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).

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 32). 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.

Table 32. HSE 4-26 MHz oscillator characteristics (1)

1. Guaranteed by design.

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

fOSC_IN

Oscillator frequency

MHz

Feedback resistor

200

Oscillator transconductance

Startup

mA/V

Gmcritmax Maximum critical crystal Gm

tSU(HSE)(2)

2. Guaranteed by characterization. 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 can vary significantly with the crystal manufacturer

Startup time

VDD is stabilized

ai17529

OSC32_IN

External

STM32F

clock source

VLSEH

tf(LSE)

tW(LSE)

90%

10 %

TLSE

tr(LSE)

tW(LSE)

fLSE_ext

VLSEL

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