Datasheet

PIC16F627A/628A/648A
DS40044G-page 100 © 2009 Microchip Technology Inc.
14.2.3 EXTERNAL CRYSTAL OSCILLATOR
CIRCUIT
Either a prepackaged oscillator can be used or a simple
oscillator circuit with TTL gates can be built.
Prepackaged 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 14-2 shows implementation of a parallel
resonant oscillator circuit. The circuit is designed to
use the fundamental frequency of the crystal. The
74AS04 inverter performs the 180° phase shift that a
parallel oscillator requires. The 4.7 kΩ resistor
provides the negative feedback for stability. The
10 kΩ potentiometers bias the 74AS04 in the linear
region. This could be used for external oscillator
designs.
FIGURE 14-2: EXTERNAL PARALLEL
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
Figure 14-3 shows a series resonant oscillator circuit.
This circuit is also designed to use the fundamental
frequency of the crystal. The inverter performs a 180°
phase shift in a series resonant oscillator circuit. The
330 kΩ resistors provide the negative feedback to bias
the inverters in their linear region.
FIGURE 14-3: EXTERNAL SERIES
RESONANT CRYSTAL
OSCILLATOR CIRCUIT
14.2.4 PRECISION INTERNAL 4 MH
Z
OSCILLATOR
The internal precision oscillator provides a fixed 4 MHz
(nominal) system clock at V
DD = 5V and 25°C. See
Section 17.0 “Electrical Specifications”, for informa-
tion on variation over voltage and temperature.
14.2.5 EXTERNAL CLOCK IN
For applications where a clock is already available
elsewhere, users may directly drive the PIC16F627A/
628A/648A provided that this external clock source meets
the AC/DC timing requirements listed in Section 17.6
“Timing Diagrams and Specifications”. Figure 14-4
below shows how an external clock circuit should be
configured.
FIGURE 14-4: EXTERNAL CLOCK INPUT
OPERATION (EC, HS, XT
OR LP OSC
CONFIGURATION)
+5V
10K
4.7K
10K
74AS04
XTAL
10K
74AS04
PIC16F627A/628A/648A
CLKIN
To o t he r
Devices
C1 C2
330 KΩ
74AS04
74AS04
PIC16F627A/
CLKIN
To o t he r
Devices
XTAL
330 KΩ
74AS04
0.1 pF
628A/648A
Clock from
ext. system
PIC16F627A/628A/648A
RA6
RA7/OSC1/CLKIN
RA6/OSC2/CLKOUT