PIC17C7XX Datasheet, PDF (17/328 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers with 10-bit A/D

4.1.3 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 TOSC).
FIGURE 4-4:
EXTERNAL CLOCK INPUT
OPERATION (EC OSC
CONFIGURATION)
Clock from
ext. system
CLKOUT
(FOSC/4)
OSC1
PIC17CXXX
OSC2
PIC17C7XX
4.1.4 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
Either a prepackaged oscillator can be used or a simple
oscillator circuit with TTL gates can be built. Prepack-
aged 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 par-
allel oscillator requires. The 4.7 kΩ resistor provides
the negative feedback for stability. The 10 kΩ potenti-
ometer biases the 74AS04 in the linear region. This
could be used for external oscillator designs.
FIGURE 4-5:
EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
+5V
10 kΩ
4.7 kΩ
74AS04
To Other
Devices
74AS04
PIC17CXXX
OSC1
10kΩ
XTAL
10 kΩ
20 pF ^{20 pF}
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
circuit. The 330 Ω resistors provide the negative feed-
back to bias the inverters in their linear region.
FIGURE 4-6:
EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
330 Ω
74AS04
330 Ω
74AS04
0.1 µF
XTAL
To Other
Devices
74AS04
PIC17CXXX
OSC1
© 1998 Microchip Technology Inc.
DS30289A-page 17