PIC16F818/819
TABLE 7-1:
CAPACITOR SELECTION FOR
THE TIMER1 OSCILLATOR
7.6
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
oscillator is a low-power oscillator, rated up to
32.768 kHz. It will continue to run during Sleep. It is
primarily intended for a 32 kHz crystal. The circuit for a
typical LP oscillator is shown in Figure 7-3. Table 7-1
shows the capacitor selection for the Timer1 oscillator.
Osc Type
Freq
C1
C2
LP
32 kHz
33 pF
33 pF
Note 1: Microchip suggests this value 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.
The user must provide a software time delay to ensure
proper oscillator start-up.
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
Note:
The Timer1 oscillator shares the T1OSI
and T1OSO pins with the PGD and PGC
pins used for programming and
debugging.
appropriate
values
of
external
components.
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.
4: Capacitor values are for design guidance
only.
7.7
Timer1 Oscillator Layout
Considerations
The Timer1 oscillator circuit draws very little power
during operation. Due to the low-power nature of the
oscillator, it may also be sensitive to rapidly changing
signals in close proximity.
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.
The oscillator circuit, shown in Figure 7-3, 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.
FIGURE 7-3:
EXTERNAL
If a high-speed circuit must be located near the oscilla-
tor, a grounded guard ring around the oscillator circuit,
as shown in Figure 7-4, may be helpful when used on
a single-sided PCB or in addition to a ground plane.
COMPONENTS FOR THE
TIMER1 LP OSCILLATOR
C1
33 pF
PIC16F818/819
T1OSI
FIGURE 7-4:
OSCILLATOR CIRCUIT
WITH GROUNDED
GUARD RING
XTAL
32.768 kHz
T1OSO
C2
VSS
33 pF
OSC1
OSC2
Note:
See the Notes with Table 7-1 for additional
information about capacitor selection.
RB7
RB6
RB5
DS39598E-page 60
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
