Datasheet

Rev. 3.00, 03/04, page 722 of 830
22.1 Oscillator
Clock pulses can be supplied either by connecting a crystal resonator or by providing external
clock input.
22.1.1 Connecting Crystal Resonator
Figure 22.2 shows a typical method of connecting a crystal resonator. An appropriate damping
resistance R
d
, given in table 22.1, should be used. An AT-cut parallel-resonance crystal resonator
should be used.
Figure 22.3 shows the equivalent circuit of a crystal resonator. A crystal resonator having the
characteristics given in table 22.2 should be used.
When PFSEL is high, the system clock (φ) frequency should be no more than 25 MHz and a
crystal resonator with frequency identical to that of the system clock (φ) should be used. When
PFSEL is low, a crystal resonator with ¼ times the frequency of the system clock (φ) should be
used.
EXTAL
XTAL
R
d
C
L2
C
L1
C
L1
= C
L2
= 10 to 22 pF
Figure 22.2 Typical Connection to Crystal Resonator
Table 22.1 Damping Resistance Values
Frequency (MHz) 5 8 10 12 16 20 25
R
d
(Ω) 300 200 0 0 0 0 0
XTAL
C
L
AT-cut parallel-resonance crystal resonato
r
EXTAL
C
0
LR
s
Figure 22.3 Equivalent Circuit of Crystal Resonator