Datasheet

 2010 Microchip Technology Inc. Preliminary DS39979A-page 35
PIC18F87J72 FAMILY
4.0 POWER-MANAGED MODES
The PIC18F87J72 family devices provide the ability to
manage power consumption by simply managing clock-
ing to the CPU and the peripherals. In general, a lower
clock frequency and a reduction in the number of circuits
being clocked constitutes lower consumed power. For
the sake of managing power in an application, there are
three primary modes of operation:
• Run mode
• Idle mode
• Sleep mode
These modes define which portions of the device are
clocked and at what speed. The Run and Idle modes
may use any of the three available clock sources
(primary, secondary or internal oscillator block); the
Sleep mode does not use a clock source.
The power-managed modes include several
power-saving features offered on previous PIC
®
devices. One is the clock switching feature, offered in
other PIC18 devices, allowing the controller to use the
Timer1 oscillator in place of the primary oscillator. Also
included is the Sleep mode, offered by all PIC devices,
where all device clocks are stopped.
4.1 Selecting Power-Managed Modes
Selecting a power-managed mode requires two
decisions: if the CPU is to be clocked or not and which
clock source is to be used. The IDLEN bit
(OSCCON<7>) controls CPU clocking, while the
SCS<1:0> bits (OSCCON<1:0>) select the clock
source. The individual modes, bit settings, clock
sources and affected modules are summarized in
Table 4-1.
4.1.1 CLOCK SOURCES
The SCS<1:0> bits allow the selection of one of three
clock sources for power-managed modes. They are:
• the primary clock, as defined by the FOSC<2:0>
Configuration bits
• the secondary clock (Timer1 oscillator)
• the internal oscillator
4.1.2 ENTERING POWER-MANAGED
MODES
Switching from one power-managed mode to another
begins by loading the OSCCON register. The
SCS<1:0> bits select the clock source and determine
which Run or Idle mode is to be used. Changing these
bits causes an immediate switch to the new clock
source, assuming that it is running. The switch may
also be subject to clock transition delays. These are
discussed in Section 4.1.3 “Clock Transitions and
Status Indicators” and subsequent sections.
Entry to the power-managed Idle or Sleep modes is
triggered by the execution of a SLEEP instruction. The
actual mode that results depends on the status of the
IDLEN bit.
Depending on the current mode and the mode being
switched to, a change to a power-managed mode does
not always require setting all of these bits. Many
transitions may be done by changing the oscillator
select bits, or changing the IDLEN bit, prior to issuing a
SLEEP instruction. If the IDLEN bit is already
configured correctly, it may only be necessary to
perform a SLEEP instruction to switch to the desired
mode.
TABLE 4-1: POWER-MANAGED MODES
Mode
OSCCON Bits Module Clocking
Available Clock and Oscillator Source
IDLEN<7>
(1)
SCS<1:0> CPU Peripherals
Sleep 0 N/A Off Off None – All clocks are disabled
PRI_RUN N/A 10 Clocked Clocked Primary – HS, EC, HSPLL, ECPLL;
this is the normal full-power execution mode
SEC_RUN N/A 01 Clocked Clocked Secondary – Timer1 Oscillator
RC_RUN N/A 11 Clocked Clocked Internal Oscillator
PRI_IDLE 110Off Clocked Primary – HS, EC, HSPLL, ECPLL
SEC_IDLE 101Off Clocked Secondary – Timer1 Oscillator
RC_IDLE 111Off Clocked Internal Oscillator
Note 1: IDLEN reflects its value when the SLEEP instruction is executed.