SX28ACSLASHDP datasheet(23/51 Pages) PARALLAX

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Parallax SX20AC/SX28AC

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Page 23 of 51

Rev 1.6 11/20/2006

9.0

OSCILLATOR CIRCUITS

The device supports several user-selectable oscillator

modes. The oscillator modes are selected by programming

the appropriate values into the FUSE Word register.

These are the different oscillator modes offered:

LP: Low Power Crystal

XT: Crystal/Resonator

HS: High Speed Crystal/Resonator

RC: External Resistor/Capacitor

9.1.

XT, LP or HS modes

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

resonator circuit or an external clock source as the device

clock.

To use an external resonator circuit, connect a crystal or

ceramic

resonator

the

OSC1/CLKIN

and

OSC2/CLKOUT

pins

according

the

circuit

configuration shown in Figure 9-1. A parallel resonant

crystal type is recommended. Use of a series resonant

crystal is not recommended. 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 5, 1 and 0 of the FUSE register (FOSC2:FOSC0) 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.

Figure 9-1: Crystal Operation

(or Ceramic Resonator)

(HS, XT or LP OSC Configuration)

Figure 9-2: Crystal Operation

(or Ceramic Resonator)

(HS, XT or LP OSC Configuration)

Table 9-1: External Component Selection for Crystal Oscillator (Vdd = 5.0V), Rs = 0 Ω

FOSC2:FOSC0

Crystal Frequency

010

4 MHz

15 pF

22 pF

1 MΩ

011

8 MHz

56 pF

33 pF

1 MΩ

011

20 MHz

33 pF

22 pF

1 MΩ

011

32 MHz

15 pF

22 pF

1 MΩ

100

50* MHz

15 pF

1 MΩ

* 50 MHz fundamental crystal

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