Datasheet

 1998-2013 Microchip Technology Inc. DS30289C-page 19
PIC17C7XX
4.1.4 EXTERNAL CLOCK OSCILLATOR
In the EC oscillator mode, the OSC1 input can be
driven by CMOS drivers. In this mode, the OSC1/
CLKIN pin is hi-impedance and the OSC2/CLKOUT pin
is the CLKOUT output (4 T
OSC).
FIGURE 4-4: EXTERNAL CLOCK INPUT
OPERATION (EC OSC
CONFIGURATION)
4.1.5 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
Either a prepackaged oscillator can be used, or a sim-
ple oscillator circuit with TTL gates can be built. Pre-
packaged oscillators provide a wide operating range
and better stability. A well designed crystal oscillator
will provide good performance with TTL gates. Two
types of crystal oscillator circuits can be used: one with
series resonance, or one with parallel resonance.
Figure 4-5 shows implementation of a parallel resonant
oscillator circuit. The circuit is designed to use the fun-
damental frequency of the crystal. The 74AS04 inverter
performs the 180-degree phase shift that a parallel
oscillator requires. The 4.7 k resistor provides the
negative feedback for stability. The 10 k potentiome-
ter biases the 74AS04 in the linear region. This could
be used for external oscillator designs.
FIGURE 4-5: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 4-6 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental fre-
quency of the crystal. The inverter performs a 180-
degree phase shift in a series resonant oscillator cir-
cuit. The 330  resistors provide the negative feedback
to bias the inverters in their linear region.
FIGURE 4-6: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Clock from
ext. system
OSC1
OSC2
PIC17CXXX
CLKOUT
(F
OSC/4)
20 pF
+5V
20 pF
10 k
4.7 k
10 k
74AS04
XTAL
10k
74AS04
PIC17CXXX
OSC1
To Other
Devices
330 
74AS04
74AS04
PIC17CXXX
OSC1
To Other
Devices
XTAL
330 
74AS04
0.1 F