PIC18F2331 Datasheet, PDF (141/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
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 oscilla-
tor is a low-power oscillator rated for 32 kHz crystals. It
will continue to run during all power-managed 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.
TABLE 12-1: CAPACITOR SELECTION FOR
THE TIMER OSCILLATOR
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.
12.3 Timer1 Oscillator Layout
Considerations
The Timer1 oscillator for PIC18F2331/2431/4331/4431
devices incorporates an additional low-power feature.
When this option is selected, it allows the oscillator to
automatically reduce its power consumption when the
microcontroller is in Sleep mode. During normal device
operation, the oscillator draws full current. As high
noise environments may cause excessive oscillator
instability in Sleep mode, this option is best suited for
low noise applications where power conservation is an
important design consideration.
The low-power option is enabled by clearing the
T1OSCMX bit (CONFIG3L<5>). By default, the option
is disabled, which results in a more-or-less constant
current draw for the Timer1 oscillator.
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
RC0
RC1
Note: Not drawn to scale.
RC2
 2003 Microchip Technology Inc.
Preliminary
DS39616B-page 139