PIC18F6520-I Datasheet, PDF (139/380 Pages) Micrel Semiconductor – 64/80-Pin High-Performance, 256 Kbit to 1 Mbit Enhanced Flash Microcontrollers with A/D

PIC18F6520/8520/6620/8620/6720/8720
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.
12.2.1 LOW-POWER TIMER1 OPTION
(PIC18FX520 DEVICES ONLY)
The Timer1 oscillator for PIC18LFX520 devices incor-
porates a low-power feature, which allows the oscillator
to automatically reduce its power consumption when
the microcontroller is in Sleep mode.
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. 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:
RC2
Not drawn to scale.
Note:
PIC18FX620/X720 devices have the
standard Timer1 oscillator permanently
selected. PIC18LFX620/X720 devices
have the low-power Timer1 oscillator
permanently selected.
 2004 Microchip Technology Inc.
DS39609B-page 137