Datasheet

PIC18F47J13 FAMILY
DS39974A-page 214 Preliminary  2010 Microchip Technology Inc.
13.5.1 USING TIMER1 AS A
CLOCK SOURCE
The Timer1 oscillator is also available as a clock source
in power-managed modes. By setting the clock select
bits, SCS<1:0> (OSCCON<1:0>), to ‘01’, the device
switches to SEC_RUN mode. Both the CPU and
peripherals are clocked from the Timer1 oscillator. If the
IDLEN bit (OSCCON<7>) is cleared and a SLEEP
instruction is executed, the device enters SEC_IDLE
mode. Additional details are available in Section 4.0
“Low-Power Modes”.
Whenever the Timer1 oscillator is providing the clock
source, the Timer1 system clock status flag,
SOSCRUN (OSCCON2<6>), is set. This can be used
to determine the controller’s current clocking mode. It
can also indicate the clock source currently being used
by the Fail-Safe Clock Monitor. If the Clock Monitor is
enabled and the Timer1 oscillator fails while providing
the clock, polling the SOSCRUN bit will indicate
whether the clock is being provided by the Timer1
oscillator or another source.
13.5.2 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. This is especially true when
the oscillator is configured for extremely Low-Power
mode (SOSCSEL = 0b01).
The oscillator circuit, displayed in Figure 13-2, should
be located as close as possible to the microcontroller.
There should be no circuits passing within the oscillator
circuit boundaries other than V
SS or VDD.
If a high-speed circuit must be located near the
oscillator (such as the ECCP1 pin in Output Compare
or PWM mode, or the primary oscillator using the
OSC2 pin), a grounded guard ring around the oscillator
circuit, as displayed in Figure 13-3, may be helpful
when used on a single-sided PCB or in addition to a
ground plane.
FIGURE 13-3: OSCILLATOR CIRCUIT
WITH GROUNDED
GUARD RING
In the Low Drive Level mode, SOSCSEL = 0b01, it is
critical that the RC2 I/O pin signals be kept away from
the oscillator circuit. Configuring RC2 as a digital out-
put, and toggling it, can potentially disturb the oscillator
circuit, even with relatively good PCB layout. If
possible, it is recommended to either leave RC2
unused, or use it as an input pin with a slew rate limited
signal source. If RC2 must be used as a digital output,
it may be necessary to use the Higher Drive Level
Oscillator mode (SOSCSEL = 0b11) with many PCB
layouts. Even in the High Drive Level mode, careful lay-
out procedures should still be followed when designing
the oscillator circuit.
In addition to dV/dt induced noise considerations, it is
also important to ensure that the circuit board is clean.
Even a very small amount of conductive soldering flux
residue can cause PCB leakage currents which can
overwhelm the oscillator circuit.
13.6 Timer1 Interrupt
The TMR1 register pair (TMR1H:TMR1L) increments
from 0000h to FFFFh and rolls over to 0000h. The
Timer1 interrupt, if enabled, is generated on overflow
which is latched in interrupt flag bit, TMR1IF
(PIR1<0>). This interrupt can be enabled or disabled
by setting or clearing the Timer1 Interrupt Enable bit,
TMR1IE (PIE1<0>).
VDD
OSC1
V
SS
OSC2
RC0
RC1
RC2
Note: Not drawn to scale.