Datasheet

© 2007 Microchip Technology Inc. DS39597C-page 61
PIC16F72
11.2 Oscillator Configurations
11.2.1 OSCILLATOR TYPES
The PIC16F72 can be operated in four different Oscil-
lator modes. The user can program two configuration
bits (FOSC1 and FOSC0) to select one of these four
modes:
• LP Low Power Crystal
• XT Crystal/Resonator
• HS High Speed Crystal/Resonator
• RC Resistor/Capacitor
11.2.2 CRYSTAL OSCILLATOR/CERAMIC
RESONATORS
In XT, LP or HS modes, a crystal or ceramic resonator
is connected to the OSC1/CLKI and OSC2/CLKO pins
to establish oscillation (Figure 11-1). The PIC16F72
oscillator design requires the use of a parallel cut crys-
tal. Use of a series cut crystal may give a frequency out
of the crystal manufacturers specifications. When in HS
mode, the device can accept an external clock source
to drive the OSC1/CLKI pin (Figure 11-2). See
Figure 14-1 or Figure 14-2 (depending on the part
number and V
DD range) for valid external clock
frequencies.
FIGURE 11-1: CRYSTAL/CERAMIC
RESONATOR OPERATION
(HS, XT OR LP
OSC CONFIGURATION)
FIGURE 11-2: EXTERNAL CLOCK INPUT
OPERATION (HS OSC
CONFIGURATION)
TABLE 11-1: CERAMIC RESONATORS
(FOR DESIGN
GUIDANCE ONLY)
Note 1: See Table 11-1 and Table 11-2 for typical
values of C1 and C2.
2: A series resistor (RS) may be required for
AT strip cut crystals.
3: RF varies with the crystal chosen.
C1
(1)
C2(1)
XTAL
OSC2
OSC1
RF(3)
SLEEP
To
logic
PIC16F72
RS
(2)
internal
Typical Capacitor Values Used:
Mode Freq OSC1 OSC2
XT 455 kHz
2.0 MHz
4.0 MHz
56 pF
47 pF
33 pF
56 pF
47 pF
33 pF
HS 8.0 MHz
16.0 MHz
27 pF
22 pF
27 pF
22 pF
Capacitor values are for design guidance only.
These capacitors were tested with the resonators
listed below for basic start-up and operation. These
values were not optimized.
Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
V
DD and temperature range for the application.
See the notes at the bottom of page 62 for additional
information.
OSC1
OSC2
Open
Clock from
Ext. System
PIC16F72
(HS Mode)