PIC18F2220 Datasheet, PDF (125/388 Pages) Microchip Technology – 28/40/44-Pin High-Performance, Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology

PIC18F2220/2320/4220/4320
12.2 Timer1 Oscillator
A crystal oscillator circuit is built-in between pins,
T1OSI (input) and T1OSO (amplifier output). It is
enabled by setting control bit, T1OSCEN (T1CON<3>).
The oscillator is a low-power oscillator rated for 32 kHz
crystals. It will continue to run during all power man-
aged modes. The circuit for a typical LP oscillator is
shown in Figure 12-3. Table 12-1 shows the capacitor
selection for the Timer1 oscillator.
The user must provide a software time delay to ensure
proper start-up of the Timer1 oscillator.
FIGURE 12-3:
C1
33 pF
EXTERNAL COMPONENTS
FOR THE TIMER1 LP
OSCILLATOR
PIC18FXXXX
T1OSI
XTAL
32.768 kHz
C2
33 pF
T1OSO
Note: See the Notes with Table 12-1 for additional
information about capacitor selection.
12.3 Timer1 Oscillator Layout
Considerations
The Timer1 oscillator circuit draws very little power
during operation. Due to the low power nature of the
oscillator, it may also be sensitive to rapidly changing
signals in close proximity.
The oscillator circuit, shown in Figure 12-3, should be
located as close as possible to the microcontroller.
There should be no circuits passing within the oscillator
circuit boundaries other than VSS or VDD.
If a high-speed circuit must be located near the oscilla-
tor (such as the CCP1 pin in output compare or PWM
mode, or the primary oscillator using the OSC2 pin), a
grounded guard ring around the oscillator circuit, as
shown in Figure 12-4, may be helpful when used on a
single-sided PCB or in addition to a ground plane.
FIGURE 12-4:
OSCILLATOR CIRCUIT
WITH GROUNDED GUARD
RING
VDD
VSS
OSC1
OSC2
TABLE 12-1: CAPACITOR SELECTION FOR
THE TIMER OSCILLATOR^(2,3,4)
Osc Type
Freq
C1
C2
LP
32 kHz
27 pF⁽¹⁾
27 pF⁽¹⁾
Note 1: Microchip suggests this value as a starting
point in validating the oscillator circuit.
2: Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate values of external
components.
4: Capacitor values are for design guidance
only.
RC0
RC1
Note: Not drawn to scale.
RC2
 2003 Microchip Technology Inc.
DS39599C-page 123