PIC18F2331/2431/4331/4431
directly to clock the system, or may be divided down
first. The INTOSC output is disabled if the system clock
is provided directly from the INTRC output.
3.7.2
OSCILLATOR TRANSITIONS
The PIC18F2331/2431/4331/4431 devices contain
circuitry to prevent clocking “glitches” when switching
between clock sources. A short pause in the system
clock occurs during the clock switch. The length of this
pause is between 8 and 9 clock periods of the new
clock source. This ensures that the new clock source is
stable and that its pulse width will not be less than the
shortest pulse width of the two clock sources.
If the Sleep mode is selected, all clock sources are
stopped. Since all the transistor switching currents
have been stopped, Sleep mode achieves the lowest
current consumption of the device (only leakage
currents).
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 system clock source (i.e., SSP slave,
INTx pins, A/D conversions and others).
Clock transitions are discussed in greater detail in
Section 4.1.2 “Entering Power-Managed Modes”.
3.8
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. The OSC1 pin (and OSC2 pin,
if used by the oscillator) will stop oscillating.
3.9
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
circumstances, and the primary clock is operating and
stable. For additional information on power-up delays,
see Section 5.3 “Power-on Reset (POR)” through
Section 5.4 “Brown-out Reset (BOR)”.
When the device executes a SLEEP instruction, the
system is switched to one of the power-managed
modes, depending on the state of the IDLEN and
SCS<1:0> bits of the OSCCON register. See
Section 4.0 “Power-Managed Modes” for details.
In secondary clock modes (SEC_RUN and
SEC_IDLE), the Timer1 oscillator is operating and
providing the system clock. The Timer1 oscillator may
also run in all power-managed modes if required to
clock Timer1.
The first timer is the Power-up Timer (PWRT), which
provides a fixed delay on power-up (parameter 33,
Table 26-8), if enabled, in Configuration Register 2L.
The second timer is the Oscillator Start-up Timer
(OST), intended to keep the chip in Reset until the crys-
tal oscillator is stable (LP, XT and HS modes). The OST
does this by counting 1024 oscillator cycles before
allowing the oscillator to clock the device.
In internal oscillator modes (RC_RUN and RC_IDLE),
the internal oscillator block provides the system clock
source. The INTRC output can be used directly to
provide the system clock and may be enabled to
support various special features, regardless of the
power-managed mode (see Section 23.2 “Watchdog
Timer (WDT)” through Section 23.4 “Fail-Safe Clock
Monitor”). The INTOSC output at 8 MHz may be used
When the HSPLL Oscillator mode is selected, the
device is kept in Reset for an additional 2 ms, following
the HS mode OST delay, so the PLL can lock to the
incoming clock frequency.
TABLE 3-3:
OSC1 AND OSC2 PIN STATES IN SLEEP MODE
OSC Mode
OSC1 Pin
OSC2 Pin
RC, INTIO1
Floating, external resistor
should pull high
At logic low (clock/4 output)
RCIO, INTIO2
Floating, external resistor
should pull high
Configured as PORTA, bit 6
ECIO
Floating, pulled by external clock
Floating, pulled by external clock
Configured as PORTA, bit 6
At logic low (clock/4 output)
EC
LP, XT and HS
Feedback inverter disabled at
quiescent voltage level
Feedback inverter disabled at
quiescent voltage level
Note: See Table 5-1 in Section 5.0 “Reset” for time-outs due to Sleep and MCLR Reset.
2010 Microchip Technology Inc.
DS39616D-page 37
MICROCHIP [ MICROCHIP ]
