Datasheet

© 2007 Microchip Technology Inc. DS39605F-page 25
PIC18F1220/1320
3.3.3 RC_IDLE MODE
In RC_IDLE mode, the CPU is disabled, but the periph-
erals continue to be clocked from the internal oscillator
block using the INTOSC multiplexer. This mode allows
for controllable power conservation during Idle periods.
This mode is entered by setting the IDLEN bit, setting
SCS1 (SCS0 is ignored) and executing a SLEEP
instruction. The INTOSC multiplexer may be used to
select a higher clock frequency by modifying the IRCF
bits before executing the SLEEP instruction. When the
clock source is switched to the INTOSC multiplexer
(see Figure 3-7), the primary oscillator is shut down
and the OSTS bit is cleared.
If the IRCF bits are set to a non-zero value (thus,
enabling the INTOSC output), the IOFS bit becomes
set after the INTOSC output becomes stable, in about
1 ms. Clocks to the peripherals continue while the
INTOSC source stabilizes. If the IRCF bits were
previously at a non-zero value before the SLEEP
instruction was executed and the INTOSC source was
already stable, the IOFS bit will remain set. If the IRCF
bits are all clear, the INTOSC output is not enabled and
the IOFS bit will remain clear; there will be no indication
of the current clock source.
When a wake event occurs, the peripherals continue to
be clocked from the INTOSC multiplexer. After a 10 μs
delay following the wake event, the CPU begins exe-
cuting code, being clocked by the INTOSC multiplexer.
The microcontroller operates in RC_RUN mode until
the primary clock becomes ready. When the primary
clock becomes ready, a clock switchback to the primary
clock occurs (see Figure 3-8). When the clock switch is
complete, the IOFS bit is cleared, the OSTS bit is set
and the primary clock is providing the system clock.
The IDLEN and SCS bits are not affected by the wake-
up. The INTRC source will continue to run if either the
WDT or the Fail-Safe Clock Monitor is enabled.
FIGURE 3-7: TIMING TRANSITION TO RC_IDLE MODE
FIGURE 3-8: TIMING TRANSITION FOR WAKE FROM RC_RUN MODE (RC_RUN TO PRI_RUN)
Q4Q3Q2
OSC1
Peripheral
Program
Q1
INTRC
Q1
Counter
Clock
CPU
Clock
PC + 2PC
12345678
Clock Transition
Q1
Q3 Q4
OSC1
Peripheral
Program
PC PC + 2
INTOSC
PLL Clock
Q1
PC + 6
Q2
Output
Q3
Q4
Q1
CPU Clock
PC + 4
Clock
Counter
Q2
Q2
Q3
Note 1: TOST = 1024 TOSC; TPLL = 2 ms (approx). These intervals are not shown to scale.
Wake from Interrupt Event
TOST
(1)
TPLL
(1)
12345678
Clock Transition
OSTS bit Set
Multiplexer
Q4