sx20ac Parallax, Inc., sx20ac Datasheet - Page 23

• Manufacturer Part Number: sx20ac
• Description: Configurable Communications Controllers With Ee/flash Program Memory, In-system Programming Capability And On-chip Debug
• Manufacturer: Parallax, Inc.

Parallax SX20AC/SX28AC
9.0
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:
9.1.
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
OSC2/CLKOUT
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
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.
* 50 MHz fundamental crystal
© Parallax Inc.
FOSC2:FOSC0
OSCILLATOR CIRCUITS
LP: Low Power Crystal
XT: Crystal/Resonator
HS: High Speed Crystal/Resonator
RC: External Resistor/Capacitor
XT, LP or HS modes
010
011
011
011
100
resonator
Table 9-1: External Component Selection for Crystal Oscillator (V
oscillator.
pins
to
according
Crystal Frequency
Table
the
50* MHz
20 MHz
32 MHz
4 MHz
8 MHz
OSC1/CLKIN
9-2
to
shows
the
circuit
and
the
Page 23 of 51
15 pF
56 pF
33 pF
15 pF
15 pF
C1
(HS, XT or LP OSC Configuration)
(HS, XT or LP OSC Configuration)
Figure 9-1: Crystal Operation
Figure 9-2: Crystal Operation
(or Ceramic Resonator)
(or Ceramic Resonator)
22 pF
33 pF
22 pF
22 pF
15 pF
C2
dd
= 5.0V), Rs = 0 Ω
Rev 1.6 11/20/2006
www.parallax.com
1 MΩ
1 MΩ
1 MΩ
1 MΩ
1 MΩ
R
F