STM32F479AI datasheet(113/208 Pages) STMICROELECTRONICS

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STM32F479xx

Electrical characteristics

187

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.

Note:

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

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

Figure 29. 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 informations given in this paragraph are based on

characterization results obtained with typical external components specified in Table 38.

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).

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) HSE accuracy

− 500

500

ppm

Gm_crit_max Maximum critical crystal gm

Startup

mA/V

tSU(HSE)(3)

Startup time

VDD is stabilized

1. Guaranteed by design.

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.

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 based on characterization and not tested in production. It is measured

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

Table 37. HSE 4-26 MHz oscillator characteristics (1) (continued)

Symbol

Parameter

Conditions

Min

Typ

Max

Unit

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