STM32H7B3AI datasheet(109/238 Pages) STMICROELECTRONICS

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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.

For CL1 and CL2, it is recommended to use high-quality external ceramic capacitors in the 5 pF to 25 pF range

(typical), designed for high-frequency applications, and selected to match the requirements of the crystal or

resonator (see Figure 32. Typical application with an 8 MHz crystal). 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. The

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 32. Typical application with an 8 MHz crystal

OSC_ OU T

OSC_ IN

fHS E

C L1

R F

STM32

8 MHz

resonator

Resonator with

integrated capacitors

Bias

controlled

gain

R EXT (1)

C L2

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 50. Low-speed external user clock characteristics. 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 50. Low-speed external user clock characteristics

Symbol

Parameter

Operating conditions(1)

Min(2)

Typ(2)

Max(2)

Unit

Oscillator frequency

32.768

kHz

IDD

LSE current consumption

LSEDRV[1:0] = 00,

Low drive capability

290

LSEDRV[1:0] = 01,

Medium Low drive capability

390

LSEDRV[1:0] = 10,

Medium high drive capability

550

LSEDRV[1:0] = 11,

High drive capability

900

Gmcritmax

Maximum critical crystal gm

LSEDRV[1:0] = 00,

Low drive capability

0.5

µA/V

LSEDRV[1:0] = 01,

Medium Low drive capability

0.75

LSEDRV[1:0] = 10,

1.7

STM32H7B3xI

Operating conditions

DS13139 - Rev 8

page 109/238

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