STM32F446VC datasheet(105/202 Pages) STMICROELECTRONICS

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STM32F446xC/E

Electrical characteristics

175

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 25). 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.

Note:

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

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

Figure 25. Typical application with an 8 MHz crystal

1. 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 40. In

the application, the resonator and the load capacitors have to be placed as close as

Table 39. 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

IDD

HSE current consumption

VDD=3.3 V,

ESR= 30 Ω,

CL=5 pF@25 MHz

450

µA

VDD=3.3 V,

ESR= 30 Ω,

CL=10 pF@25 MHz

530

ACCHSE(2)

2. This parameter depends on the crystal used in the application. The minimum and maximum values must

be respected to comply with USB standard specifications.

HSE accuracy

-500

500

ppm

Gm_crit_max Maximum critical crystal gm

Startup

mA/V

tSU(HSE(3)

3. 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 Guaranteed based on test during characterization. It is measured for a

standard crystal resonator and it can vary significantly with the crystal manufacturer.

Startup time

VDD is stabilized

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