Datasheet

33
3286P–MICRO–3/10
AT89S8253
16. Oscillator Characteristics
XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier that can be
configured for use as an on-chip oscillator, as shown in Figure 16-1 (A) and (B). Either a quartz
crystal or ceramic resonator may be used. For frequencies above 16MHz it is recommended that
C1 be replaced with R1 for improved startup performance. Note that the internal structure of the
devices adds about 10 pF of capacitance to both XTAL1 and XTAL2. The total capacitance on
XTAL1 or XTAL2, including the external load capacitor (C1/C2) plus internal device load, board
trace and crystal loadings, should not exceed 20 pF. Figure 16-2, 16-3, 16-4 and 16-5 illustrate
the relationship between clock loading and the respective resulting clock amplitudes.
Figure 16-1. Oscillator Connections
Note: C1, C2 = 0–10 pF for Crystals
= 0–10 pF for Ceramic Resonators
R1 = 4–5 MΩ
Figure 16-2. Quartz Crystal Clock Source (A)
~10 pF
~10 pF
C2
C1
~10 pF
~10 pF
C2
R1
(A) Low Frequency (B) High Frequency
Quartz Crystal Clock Input
0
1
2
3
4
5
6
04812162024
Frequency (MHz)
XTAL1 Amplitude (V)
C1=C2=0pF
C1=C2=5pF
C1=C2=10pF