STA321 datasheet(18/157 Pages) STMICROELECTRONICS

18 / 157 page

Electrical specifications

STA321

18/157

Doc ID 15351 Rev 3

The STA321 has an integrated oscillator between pins XTI and XTO.

The architecture is a single-stage oscillator with an inverter working as an amplifier. The

oscillator stage is biased by an internal resistor (of about 500 k

Ω), and requires an external

PI network consisting of a crystal and two capacitors as shown in Figure 4 below. An enable

feature is provided in bit 7 of register MISC (address 0xC8) to stop the oscillator and thereby

to reduce power consumption.

Not all crystals operate satisfactorily with the type of oscillator used in the STA321. To find

out if a crystal is suitable for this device the following transconductance formula must be

evaluated and compared to the critical transconductance for the embedded oscillator:

Gm = Rm *

ω2 * (C + 2 * Co)2 < GmCRITICAL / 3

where

ω is the crystal operating frequency, C = CA = CB, Co and Rm are shown in Figure 5

and GmCRITICAL is given in Table 7.

Figure 5.

Equivalent circuit of crystal and external components

Table 7.

Oscillator specifications

Symbol

Parameter

Min

Typ

Max

Unit

IOSC

Oscillator power consumption with crystal

connected (1)

If no crystal is connected then the power consumption could be much higher.

--215

µA

DutyOSC

Duty cycle

46.9

47.8%

48.9

TUP

Startup time

15 *

τx-

s (2)

τx is the time constant of the crystal and external components; a typical value is 44 µs.

GmCRITICAL

Oscillator transconductance

1060

µA/V

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