Datasheet

PIC18F1220/1320
DS39605F-page 18 © 2007 Microchip Technology Inc.
2.7.2 OSCILLATOR TRANSITIONS
The PIC18F1220/1320 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.
Clock transitions are discussed in greater detail in
Section 3.1.2 “Entering Power Managed Modes”.
2.8 Effects of Power Managed Modes
on the Various Clock Sources
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
SCS1:SCS0 bits of the OSCCON register. See
Section 3.0 “Power Managed Modes” for details.
When PRI_IDLE mode is selected, the designated pri-
mary 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.
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 or Timer3.
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 19.2 “Watchdog
Timer (WDT)” through Section 19.4 “Fail-Safe Clock
Monitor”). The INTOSC output at 8 MHz may be used
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.
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., INTn pins, A/D
conversions and others).
2.9 Power-up Delays
Power-up delays are controlled by two timers, so that
no external Reset circuitry is required for most applica-
tions. The delays ensure that the device is kept in
Reset until the device power supply is stable under nor-
mal circumstances and the primary clock is operating
and stable. For additional information on power-up
delays, see Sections 4.1 through 4.5.
The first timer is the Power-up Timer (PWRT), which
provides a fixed delay on power-up (parameter 33,
Table 22-8) 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
crystal 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.
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.
There is a delay of 5 to 10 μs 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,
RC or INTIO modes are used as the primary clock
source.
TABLE 2-3: OSC1 AND OSC2 PIN STATES IN SLEEP MODE
Oscillator 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 Configured as PORTA, bit 6
EC Floating, pulled by external clock At logic low (clock/4 output)
LP, XT and HS Feedback inverter disabled at quiescent
voltage level
Feedback inverter disabled at quiescent
voltage level
Note: See Table 4-1 in Section 4.0 “Reset” for time-outs due to Sleep and MCLR
Reset.