SX28AC75-ISLASHTQ datasheet(21/52 Pages) ETC

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SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75

9.0

OSCILLATOR CIRCUITS

The device supports several user-selectable oscillator

modes. The oscillator modes are selected by program-

ming the appropriate values into the FUSE Word register.

These are the different oscillator modes offered:

9.1

XT, LP or HS modes

In XT, LP or HS, modes, you can use either an external

resonator network or an external clock signal as the

device clock.

To use an external resonator network, you connect a

crystal or ceramic resonator to the OSC1/CLKIN and

OSC2/CLKOUT pins according to the circuit configura-

tion shown in Figure 9-1. A parallel resonant crystal type

is recommended. Use of a series resonant crystal may

result in a frequency that is outside the crystal manufac-

turer specifications. Table 9-1 shows the recommended

external components associated with a crystal-based

oscillator. Table 9-2 shows the recommended external

component values for a resonator-based oscillator.

Bits 0, 1 and 5 of the FUSE register (FOSC1:FOSC2) are

used to configure the different external resonator/crystal

oscillator modes. These bits allow the selection of the

appropriate gain setting for the internal driver to match

the desired operating frequency. If the XT, LP, or HS

mode is selected, the OSC1/CLKIN pin can be driven by

an external clock source rather than a resonator network,

as long as the clock signal meets the specified duty

cycle, rise and fall times, and input levels (Figure 9-2). In

this case, the OSC2/CLKOUT pin should be left open.

LP: Low Power Crystal

XT: Crystal/Resonator

HS: High Speed Crystal/Resonator

RC: External Resistor/Capacitor

Internal Resistor/Capacitor

Figure 9-1. Crystal Operation (or Ceramic Resonator)

(HS, XT or LP OSC Configuration)

Figure 9-2. External Clock Input Operation

(HS, XT or LP OSC Configuration)

SX Device

XTAL

OSC2

OSC1

Internal

Circuitry

SLEEP

Externally

Generated Clock

OSC1

OSC2

Open

SX Device

Table 9-1. External Component Selection for Crystal Oscillator(Vdd=5.0V)

FOSC2:FOSC0

Crystal

Frequency

010

4 MHz

15 pF

22 pF

1 MΩ

Ω

011

8 MHz

56 pF

33 pF

1 MΩ

Ω

011

20 MHz

33 pF

22 pF

1 MΩ

Ω

011

32 MHz

15 pF

22 pF

1 MΩ

Ω

100

50* MHz

15 pF

1 MΩ

Ω

* 50 MHz fundamental crystal

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