PIC18F2331/2431/4331/4431
13.2 Timer1 Oscillator
13.3 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 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 13-3. Table 13-1 shows the capacitor selection
for the Timer1 oscillator.
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 user must provide a software time delay to ensure
proper start-up of the Timer1 oscillator.
FIGURE 13-3:
EXTERNAL COMPONENTS
FOR THE TIMER1 LP
OSCILLATOR
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.
C1
27 pF
PIC18FXXXX
Due to the low-power nature of the oscillator, it may
also be sensitive to rapidly changing signals in close
proximity.
T1OSI
XTAL
32.768 kHz
The oscillator circuit, shown in Figure 13-3, 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. Refer to
Section 2.0 “Guidelines for Getting Started with
PIC18F Microcontrollers” for additional information
T1OSO
C2
27 pF
Note:
See the notes with Table 13-1 for additional
information about capacitor selection.
TABLE 13-1: 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 stabil-
ity 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.
2010 Microchip Technology Inc.
DS39616D-page 133
MICROCHIP [ MICROCHIP ]
