STA8090FG datasheet(32/41 Pages) STMICROELECTRONICS

32 / 41 page

Electrical characteristics

STA8090FG

32/41

DS10719 Rev 6

4.8.2

Oscillator electrical specifications

This device contains two oscillators:



a 32.768 kHz oscillator/buffer for RTC circuit.



an OSCI oscillator/buffer in the RF Front-End

When used in oscillator mode, each oscillator requires a specific crystal, with parameters

that must be as close as possible to the following recommended values. When used in input

buffer mode, an external clock source must be applied.

32.768 kHz OSCI32 oscillator specifications

The 32.768 kHz OSCI32 oscillator is connected between RTC_XTI (oscillator amplifier

input) and RTC_XTO (oscillator amplifier output). It also requires two external capacitors of

18 pF(b), as shown on Figure 2.

OSCI32 is disabled by default and must be enabled by setting bit28-OSCI_EN of

PRCC_BACKUP_REG0 to have 32.768KHz oscillation when an XTAL pi-network is

connected to RTC_XTI/RTC_XTO pins.

The recommended oscillator specifications are shown in Table 26:

The oscillator amplifier specifications are shown in the following table:

b. Using crystal with recommended characteristics as per Table 26.

Table 26. Crystal recommended specifications

Symbol

Parameter

Min.

Typ.

Max.

Unit

FSXTAL

Crystal frequency(1)

1. Not tested in production.

—

32.768

—

kHz

LMSXTAL

Motion inductance(1)

—5

—

CMSXTAL

Motional capacitance(1)

—5.0

—

COSXTAL

Shunt capacitance(1)

—1.3

—

ESR

Resonance resistance(1)

——

kΩ

External load capacitance(1)

—18

—

Table 27. Oscillator amplifier specifications

Symbol

Parameter

Min.

Typ.

Max.

Unit

Startup time(1)

1. Not tested in production.

—0.3

0.6

Drive level(1)

——

<0.1

µW

RLC

Required load capacitance(1)

—

12.5

—

Startup transconductance

22.5

33.6

—

µA/V

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