Datasheet

PIC18F87J50 FAMILY
DS39775C-page 46 © 2009 Microchip Technology Inc.
2.6 Effects of Power-Managed Modes
on the Various Clock Sources
When PRI_IDLE mode is selected, the designated
primary oscillator continues to run without interruption.
For all other power-managed modes, the oscillator
using the OSC1 pin is disabled. Unless the USB
module is enabled, the OSC1 pin (and OSC2 pin if
used by the oscillator) will stop oscillating.
In secondary clock modes (SEC_RUN and
SEC_IDLE), the Timer1 oscillator is operating and
providing the device clock. The Timer1 oscillator may
also run in all power-managed modes if required to
clock Timer1 or Timer3.
In internal oscillator modes (RC_RUN and RC_IDLE),
the internal oscillator block provides the device clock
source. The 31 kHz INTRC output can be used directly
to provide the clock and may be enabled to support
various special features regardless of the
power-managed mode (see Section 25.2 “Watchdog
Timer (WDT)”, Section 25.4 “Two-Speed Start-up”
and Section 25.5 “Fail-Safe Clock Monitor” for more
information on WDT, Fail-Safe Clock Monitor and
Two-Speed Start-up). The INTOSC output at 8 MHz
may be used directly to clock the device or may be
divided down by the postscaler. The INTOSC output is
disabled if the clock is provided directly from the INTRC
output.
If the Sleep mode is selected, all clock sources which
are no longer required are stopped. Since all the tran-
sistor switching currents have been stopped, Sleep
mode achieves the lowest current consumption of the
device (only leakage currents).
Sleep mode should not be invoked while the USB mod-
ule is enabled and operating in full-power mode. Before
Sleep mode is selected, the USB module should be put
in the suspend state. This is accomplished by setting
the SUSPND bit in the UCON register.
Enabling any on-chip feature that will operate during
Sleep will increase the current consumed during Sleep.
The INTRC is required to support WDT operation. The
Timer1 oscillator may be operating to support a
Real-Time Clock. Other features may be operating that
do not require a device clock source (i.e., MSSP slave,
PMP, INTx pins and others). Peripherals that may add
significant current consumption are listed in
Section 28.2 “DC Characteristics: Power-Down and
Supply Current”.
2.7 Power-up Delays
Power-up delays are controlled by two timers so that no
external Reset circuitry is required for most applications.
The delays ensure that the device is kept in Reset until
the device power supply is stable under normal circum-
stances and the primary clock is operating and stable.
For additional information on power-up delays, see
Section 4.6 “Power-up Timer (PWRT)”.
The first timer is the Power-up Timer (PWRT), which
provides a fixed delay on power-up (parameter 33,
Table 28-13).
The second timer is the Oscillator Start-up Timer
(OST), intended to keep the chip in Reset until the
crystal oscillator is stable (HS mode). The OST does
this by counting 1024 oscillator cycles before allowing
the oscillator to clock the device.
There is a delay of interval, T
CSD (parameter 38,
Table 28-13), following POR, while the controller
becomes ready to execute instructions. This delay runs
concurrently with any other delays. This may be the
only delay that occurs when any of the EC or internal
oscillator modes are used as the primary clock source.