Intel 8253 - Using an External Clock; Power Control

XA User Guide 4-12 3/24/97
4.5 Oscillator
The XA contains an on-chip oscillator which may be used as the clock source for the XA CPU,
or an external clock source may be used. A quartz crystal or ceramic resonator may be connected
as shown in Figure 4.8a to use the internal oscillator. To use an external clock, connect the
source to pin XTAL1 and leave pin XTAL2 open, as shown in Figure 4.8b.
The on-chip oscillator of the XA consists of a single stage linear inverter intended for use as a
positive reactance oscillator. In this application, the crystal is operated in its fundamental
response mode as an inductive reactance in parallel resonance with capacitance external to the
crystal.
A quartz crystal or ceramic resonator is connected between the XTAL1 and XTAL2 pins,
capacitors ar connected from both pins to ground. In the case of a quartz crystal, a parallel
resonant crystal must be used in order to obtain reliable operation. The capacitor values used in
the oscillator circuit should normally be those recommended by the crystal or resonator
manufacturer. For crystals, the values may generally be from 18 to 24 pF for frequencies above
25 MHz and 28 to 34 pF for lower frequencies. Too large or too small capacitor values may
prevent oscillator start-up or adversely affect oscillator start-up time.
4.6 Power Control
The XA CPU implements two modes of reduced power consumption: Idle mode, for moderate
power savings, and Power-Down mode. Power-Down reduces XA consumption to a bare
minimum. These modes are initiated by writing SFR PCON, as illustrated in Figure 4.9.
Idle Mode is activated by setting the PCON bit IDL. This stops CPU execution while leaving the
oscillator and some peripherals running.
Figure 4.8 XA clock sources
Figure 4.9 PCON
XA
XTAL2
a) using the on-chip oscillator
XA
XTAL2
b) using an external clock
nc
XTAL1
XTAL1
C1
C2
- - - - - - PD IDL
PCON