STLUX385A datasheet(81/99 Pages) STMICROELECTRONICS

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STLUX385A

Electrical characteristics

Figure 18. HSE oscillator circuit diagram

The crystal characteristics have to be checked with the following formula:

where gm

Critic

is calculated with the crystal parameters as follows:

and where:

•

Rm: motional resistance

(b)

•

Lm: motional inductance

(b)

•

Cm: motional capacitance

(b)

•

CO: shunt capacitance

(b)

•

CL1 = CL2 = C: grounded external capacitance

Oscillator

transconductance

5mA/V

SU(HSE)

(4)

Startup time

is stabilized

2.8

In case f

HSE

is configured as direct clock for the SMED logic the maximum frequency can be 24 MHz

The oscillator needs two load capacitors, CL1 and CL2, to act as load for the crystal. The total load

capacitance (Cload) is (CL1 * CL2)/(CL1 + CL2). If CL1 = CL2, Cload = CL1 / 2. Some oscillators have

built-in load capacitors, CL1 and CL2.

The oscillator selection can be optimized in terms of supply current using a high quality resonator with

small Rm value.

SU(HSE)

is the start-up time measured from the moment it is enabled (by software) to a stabilized 16 MHz

oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly

with the crystal manufacturer.

Refer to application crystal specification.

Table 42. HSE crystal/ceramic resonator oscillator (continued)

Symbol

Parameter

Conditions

Min.

Typ.

Max.

Unit

HseOscout

HseOscin

HseOscout

GIPD100520131424FSR

mC ri t i c

mC r i ti c

π f

HS E

⋅⋅

()2R

()2

⋅

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