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18.2
Oscillator Circuit
Clock pulses can be supplied by connecting a crystal resonator, or by input of an external clock
signal.
18.2.1
Connecting a Crystal Resonator
Circuit Configuration: A crystal resonator can be connected as in the example in figure 18.2.
The damping resistance Rd should be selected according to table 18.1. An AT-cut parallel-
resonance crystal should be used.
EXTAL
XTAL
Figure 18.2 Connection of Crystal Resonator (Example)
Table 18.1 Damping Resistance Value
Damping
Resistance
2 < f ≤ 4 4 < f ≤ 8 8 < f ≤ 1 0 10 < f ≤ 13 13 < f ≤ 16 16 < f ≤ 18 18 < f ≤ 20
Value
2
Rd (Ω)
1 k
500
Note: A crystal resonator between 2 MHz and 20 MHz can be used. If the chip is to be operated
at less than 2 MHz, the on-chip frequency divider should be used. (A crystal resonator of
less than 2 MHz cannot be used.)
Crystal Resonator: Figure 18.3 shows an equivalent circuit of the crystal resonator. The crystal
resonator should have the characteristics listed in table 18.2.
550
C
L1
Rd
C
L2
Frequency f (MHz)
200
0
0
C
= C
= 10 pF to 22 pF
L1
L2
0
0
0
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