Datasheet

© 2007 Microchip Technology Inc. DS39599G-page 35
PIC18F2220/2320/4220/4320
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 previ-
ously 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-up event occurs, the peripherals con-
tinue to be clocked from the INTOSC multiplexer. After
a 10 μs delay following the wake-up event, the CPU
begins executing 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 switch back 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-up from Interrupt Event
TOST
(1)
TPLL
(1)
12345678
Clock Transition
OSTS bit Set
Multiplexer
Q4