PIC18FXX20
12.2.1
LOW POWER TIMER1 OPTION
(PIC18FX520 DEVICES ONLY)
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 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 user must provide a software time delay to ensure
proper start-up of the Timer1 oscillator
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.
FIGURE 12-3:
EXTERNAL
COMPONENTS FOR THE
TIMER1 LP OSCILLATOR
C1
PIC18FXX20
T1OSI
33 pF
XTAL
32.768 kHz
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.
T1OSO
C2
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.
33 pF
Note:
See the Notes with Table 12-1 for additional
information about capacitor selection.
TABLE 12-1: CAPACITOR SELECTION FOR
THE ALTERNATE
FIGURE 12-4:
OSCILLATOR CIRCUIT
WITH GROUNDED
GUARD RING
OSCILLATOR
Osc Type
Freq
C1
TBD(1)
C2
TBD(1)
LP
32 kHz
VDD
Crystal to be Tested:
VSS
32.768 kHz Epson C-001R32.768K-A ± 20 PPM
OSC1
OSC2
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.
RC0
RC1
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
RC2
appropriate
components.
values
of
external
Note: Not drawn to scale.
4: Capacitor values are for design guidance
only.
2003 Microchip Technology Inc.
Advance Information
DS39609A-page 137
MICROCHIP [ MICROCHIP ]
