PIC18F2220/2320/4220/4320
12.2 Timer1 Oscillator
12.3 Timer1 Oscillator Layout
Considerations
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 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.
The user must provide a software time delay to ensure
proper start-up of the Timer1 oscillator.
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 12-3:
EXTERNAL COMPONENTS
FOR THE TIMER1 LP
OSCILLATOR
C1
33 pF
PIC18FXXXX
T1OSI
FIGURE 12-4:
OSCILLATOR CIRCUIT
WITH GROUNDED GUARD
RING
XTAL
32.768 kHz
VDD
VSS
T1OSO
C2
33 pF
OSC1
OSC2
Note:
See the Notes with Table 12-1 for additional
information about capacitor selection.
RC0
RC1
TABLE 12-1: CAPACITOR SELECTION FOR
THETIMEROSCILLATOR(2,3,4)
Osc Type
Freq
C1
C2
27 pF(1)
27 pF(1)
RC2
LP
32 kHz
Note: Not drawn to scale.
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.
2003 Microchip Technology Inc.
DS39599C-page 123
MICROCHIP [ MICROCHIP ]
