PIC16F628-20P Datasheet, PDF (95/170 Pages) Microchip Technology – FLASH-Based 8-Bit CMOS Microcontroller

14.2 Oscillator Configurations
14.2.1 OSCILLATOR TYPES
The PIC16F62X can be operated in eight different
oscillator options. The user can program three
configuration bits (FOSC2 thru FOSC0) to select one of
these eight modes:
• LP
• XT
• HS
• ER
• INTRC
• EC
Low Power Crystal
Crystal/Resonator
High Speed Crystal/Resonator
External Resistor (2 modes)
Internal Resistor/Capacitor (2 modes)
External Clock In
14.2.2 CRYSTAL OSCILLATOR / CERAMIC
RESONATORS
In XT, LP or HS modes a crystal or ceramic resonator
is connected to the OSC1 and OSC2 pins to establish
oscillation (Figure 14-1). The PIC16F62X oscillator
design requires the use of a parallel cut crystal. Use of
a series cut crystal may give a frequency out of the
crystal manufacturers specifications. When in XT, LP or
HS modes, the device can have an external clock
source to drive the OSC1 pin (Figure 14-4).
FIGURE 14-1:
CRYSTAL OPERATION
(OR CERAMIC
RESONATOR) (HS, XT OR
LP OSC
CONFIGURATION)
OSC1
C1
XTAL
RF
OSC2
RS
C2
NOTE 1
SLEEP
FOSC
PIC16F62X
Note
1: A series resistor may be required for some crys-
tals.
2: See Table 14-1 and Table 14-2 for recommended
values of C1 and C2.
PIC16F62X
TABLE 14-1: CAPACITOR SELECTION FOR
CERAMIC RESONATORS
Ranges Characterized:
Mode
Freq
OSC1(C1)
OSC2(C2)
XT
455 kHz
22 - 100 pF
22 - 100 pF
2.0 MHz
15 - 68 pF
15 - 68 pF
4.0 MHz
15 - 68 pF
15 - 68 pF
HS
8.0 MHz
16.0 MHz
10 - 68 pF
10 - 22 pF
10 - 68 pF
10 - 22 pF
Note 1: Higher capacitance increases the stability of the oscilla-
tor but also increases the start-up time. These values
are for design guidance only. Since each resonator has
its own characteristics, the user should consult the res-
onator manufacturer for appropriate values of external
components.
TABLE 14-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
Mode
Freq
OSC1(C1)
OSC2(C2)
LP
32 kHz
68 - 100 pF
68 - 100 pF
200 kHz
15 - 30 pF
15 - 30 pF
XT
100 kHz
68 - 150 pF
150 - 200 pF
2 MHz
15 - 30 pF
15 - 30 pF
4 MHz
15 - 30 pF
15 - 30 pF
HS
8 MHz
15 - 30 pF
15 - 30 pF
10 MHz
15 - 30 pF
15 - 30 pF
20 MHz
15 - 30 pF
15 - 30 pF
Note 1: Higher capacitance increases the stability of the oscilla-
tor but also increases the start-up time. These values
are for design guidance only. Rs may be required in HS
mode as well as XT mode to avoid overdriving crystals
with low drive level specification. Since each crystal
has its own characteristics, the user should consult the
crystal manufacturer for appropriate values of external
components.
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.
 2003 Microchip Technology Inc.
Preliminary
DS40300C-page 93