STA1295 datasheet(85/109 Pages) STMICROELECTRONICS

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DS12586 Rev 5

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STA1295, STA1275

Electrical characteristics

106

There are two ways to drive the 32.768 kHz crystal pins from an external clock source:

•

Bypass mode (for test). Enable the bypass mode (bit XCOSC32K_BYPASS= 1b in

PMU_CTRL register). Apply external single ended clock at M3_SXTALI. Input clock

should be of CMOS level (Low = GND, High = VDDIO_ON).

•

Force Through Mode. Apply external single ended clock at M3_SXTALI. Input clock

should be of CMOS level (Low = GND, High = VDDIO_ON). Clock is available after

OSC startup time. The node M3_SXTALO must not be tied high as this may cause

large current to enter amplifier and damage it permanently.

4.8.3

24-26 MHz oscillator specifications

The internal oscillator amplifier specifications are shown in the table below.

The 24 to 26 MHz oscillator is connected between MXTALI (oscillator amplifier input) and

MXTALO (oscillator amplifier output). It also requires two external load capacitors of CL pF,

as shown in Figure 5.

The specifications of a typical external crystal are shown in the table below.

Table 41. Oscillator amplifier specifications

Symbol

Parameter

Limit values

Unit

Min. Typ. Max.

TMXTAL

Startup time

3.4

Tduty cycle(Zi & NZi) V

Duty cycle

40% 50% 60%

—

Amplitude of oscillation at MXTALO

0.4

1.6

PSXTAL

Power consumption during stable oscillation

—

μA

GM0-MXTAL

Transconductance

8.5

15.8 mA/V

Rneg

Negative resistance

175

285

Frequency stability

PPM

Table 42. Typical crystal recommended specifications

Symbol

Parameter

Limit values

Unit

Min.

Typ.

Max.

FMXTAL

Crystal frequency

MHz

LMMXTAL, 24 MHz XTAL

—

4.0

CMMXTAL, 24 MHz XTAL

—

10.1

—

RMMXTAL

Motional resistance

—

COMXTAL

Shunt capacitance

—

4.0

CLMXTAL

Load capacitance(1)

1. Total capacitance, including package and board parasitics.

——

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