PIC18FXX20 Datasheet, PDF (139/366 Pages) Microchip Technology – 64/80-Pin High Performance 1 Mbit Enhanced FLASH Microcontrollers with A/D

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 oscil-
lator is a low power oscillator rated up to 200 kHz. It will
continue to run during SLEEP. It is primarily intended
for a 32 kHz crystal. The circuit for a typical LP oscilla-
tor 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
PIC18FXX20
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 ALTERNATE
OSCILLATOR
Osc Type
Freq
C1
LP
32 kHz
TBD⁽¹⁾
Crystal to be Tested:
32.768 kHz Epson C-001R32.768K-A
C2
TBD⁽¹⁾
± 20 PPM
Note 1: Microchip suggests 33 pF 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.
PIC18FXX20
12.2.1
LOW POWER TIMER1 OPTION
(PIC18FX520 DEVICES ONLY)
The Timer1 oscillator for PIC18FX520 devices incorpo-
rates 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 environ-
ments 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 (CONFIG3H<1>). 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.
Advance Information
DS39609A-page 137