PIC16F627A Datasheet, PDF (97/168 Pages) Microchip Technology – FLASH-Based 8-Bit CMOS Microcontrollers

PIC16F627A/628A/648A
14.2 Oscillator Configurations
14.2.1 OSCILLATOR TYPES
The PIC16F627A/628A/648A can be operated in eight
different oscillator options. The user can program three
configuration bits (FOSC2 through FOSC0) to select
one of these eight modes:
• LP
Low Power Crystal
• XT
Crystal/Resonator
• HS
High Speed Crystal/Resonator
• RC External Resistor/Capacitor (2 modes)
• INTOSC Internal Precision Oscillator (2 modes)
• EC
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 PIC16F627A/628A/648A
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
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
SLEEP
OSC2
RS⁽¹⁾
C2
FOSC
PIC16F627A/628A/648A
Note
1: A series resistor may be required for AT strip cut
crystals.
2: See Table 14-1 and Table 14-2 for recommended
values of C1 and C2.
TABLE 14-1: CAPACITOR SELECTION FOR
CERAMIC RESONATORS
Mode
Freq
OSC1(C1)
OSC2(C2)
XT
HS
Note:
455 kHz
2.0 MHz
4.0 MHz
22 - 100 pF
15 - 68 pF
15 - 68 pF
22 - 100 pF
15 - 68 pF
15 - 68 pF
8.0 MHz
16.0 MHz
10 - 68 pF
10 - 22 pF
10 - 68 pF
10 - 22 pF
Higher capacitance increases the stability of the oscil-
lator 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
XT
HS
Note:
32 kHz
200 kHz
15 - 30 pF
0 - 15 pF
15 - 30 pF
0 - 15 pF
100 kHz
2 MHz
4 MHz
68 - 150 pF
15 - 30 pF
15 - 30 pF
150 - 200 pF
15 - 30 pF
15 - 30 pF
8 MHz
10 MHz
20 MHz
15 - 30 pF
15 - 30 pF
15 - 30 pF
15 - 30 pF
15 - 30 pF
15 - 30 pF
Higher capacitance increases the stability of the oscil-
lator but also increases the start-up time. These values
are for design guidance only. A series resistor (RS)
may be required in HS mode as well as XT mode to
avoid overdriving crystals with low drive level specifica-
tion. Since each crystal has its own characteristics, the
user should consult the crystal manufacturer for appro-
priate 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 reso-
nant 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.
 2002 Microchip Technology Inc.
Preliminary
DS40044A-page 95