PIC18F1220_07 Datasheet, PDF (107/308 Pages) Microchip Technology – 18/20/28-Pin High-Performance, Enhanced Flash Microcontrollers with 10-bit A/D and nanoWatt Technology

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 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.
Note:
The Timer1 oscillator shares the T1OSI
and T1OSO pins with the PGD and PGC
pins used for programming and
debugging.
When using the Timer1 oscillator, In-Circuit
Serial Programming (ICSP) may not
function correctly (high voltage or low
voltage), or the In-Circuit Debugger (ICD)
may not communicate with the controller.
As a result of using either ICSP or ICD, the
Timer1 crystal may be damaged.
If ICSP or ICD operations are required, the
crystal should be disconnected from the
circuit (disconnect either lead), or installed
after programming. The oscillator loading
capacitors may remain in-circuit during
ICSP or ICD operation.
PIC18F1220/1320
FIGURE 12-3:
EXTERNAL
COMPONENTS FOR THE
TIMER1 LP OSCILLATOR
C1
22 pF PGD
PIC18FXXXX
PGD/T1OSI
XTAL
32.768 kHz
C2 PGC
22 pF
PGC/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
22 pF⁽¹⁾
22 pF⁽¹⁾
Note 1: Microchip suggests this value as a starting
point in validating the oscillator circuit.
Oscillator operation should then be tested
to ensure expected performance under
all expected conditions (VDD and
temperature).
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.
© 2007 Microchip Technology Inc.
DS39605F-page 105