P12C508 Datasheet, PDF (26/84 Pages) Microchip Technology – 8-Pin, 8-Bit CMOS Microcontroller

PIC12C5XX
7.2 Oscillator Configurations
7.2.1 OSCILLATOR TYPES
The PIC12C5XX can be operated in four different
oscillator modes. The user can program two
configuration bits (FOSC1:FOSC0) to select one of
these four modes:
• LP: Low Power Crystal
• XT:
Crystal/Resonator
• INTRC: Internal 4 MHz Oscillator
• EXTRC: External Resistor/Capacitor
7.2.2 CRYSTAL OSCILLATOR / CERAMIC
RESONATORS
In XT or LP modes, a crystal or ceramic resonator is
connected to the GP5/OSC1/CLKIN and GP4/OSC2
pins to establish oscillation (Figure 7-2). The
PIC12C5XX 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 or LP modes, the device
can have an external clock source drive the GP5/
OSC1/CLKIN pin (Figure 7-3).
FIGURE 7-2:
C1⁽¹⁾
CRYSTAL OPERATION (OR
CERAMIC RESONATOR) (XT
OR LP OSC
CONFIGURATION)
OSC1
PIC12C5XX
XTAL
OSC2
RS⁽²⁾
C2⁽¹⁾
RF⁽³⁾
SLEEP
To internal
logic
Note 1: See Capacitor Selection tables for
recommended 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
(approx. value = 10 MΩ).
FIGURE 7-3:
EXTERNAL CLOCK INPUT
OPERATION (XT OR LP OSC
CONFIGURATION)
Clock from
ext. system
Open
OSC1
PIC12C5XX
OSC2
TABLE 7-1:
CAPACITOR SELECTION
FOR CERAMIC RESONATORS
- PIC12C5XX
Osc Resonator Cap. Range Cap. Range
Type
Freq
C1
C2
XT 455 kHz
2.0 MHz
4.0 MHz
68-100 pF
15-33 pF
10-22 pF
68-100 pF
15-33 pF
10-22 pF
These values are for design guidance only. Since
each resonator has its own characteristics, the user
should consult the resonator manufacturer for
appropriate values of external components.
TABLE 7-2:
CAPACITOR SELECTION
FOR CRYSTAL OSCILLATOR
- PIC12C5XX
Osc Resonator Cap.Range Cap. Range
Type
Freq
C1
C2
LP
32 kHz⁽¹⁾
15 pF
15 pF
XT
100 kHz 15-30 pF 200-300 pF
200 kHz 15-30 pF 100-200 pF
455 kHz 15-30 pF 15-100 pF
1 MHz
15-30 pF
15-30 pF
2 MHz
15 pF
15 pF
4 MHz
15 pF
15 pF
Note 1: For VDD > 4.5V, C1 = C2 ≈ 30 pF is
recommended.
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.
DS40139A-page 26
Advanced Information
© 1996 Microchip Technology Inc.