PIC18F2220/2320/4220/4320
2.7.1
OSCILLATOR CONTROL REGISTER
2.7
Clock Sources and Oscillator
Switching
The OSCCON register (Register 2-2) controls several
aspects of the system clock’s operation, both in full
power operation and in power managed modes.
Like previous PIC18 devices, the PIC18F2X20 and
PIC18F4X20 devices include a feature that allows the
system clock source to be switched from the main
oscillator to an alternate low-frequency clock source.
PIC18F2X20/4X20 devices offer two alternate clock
sources. When enabled, these give additional options
for switching to the various power managed operating
modes.
The System Clock Select bits, SCS1:SCS0, select the
clock source that is used when the device is operating
in power managed modes. The available clock sources
are the primary clock (defined in Configuration
Register 1H), the secondary clock (Timer1 oscillator)
and the internal oscillator block. The clock selection
has no effect until a SLEEPinstruction is executed and
the device enters a power managed mode of operation.
The SCS bits are cleared on all forms of Reset.
Essentially, there are three clock sources for these
devices:
• Primary oscillators
The Internal Oscillator Select bits, IRCF2:IRCF0, select
the frequency output of the internal oscillator block that
is used to drive the system clock. The choices are the
INTRC source, the INTOSC source (8 MHz) or one of
the six frequencies derived from the INTOSC
postscaler (125 kHz to 4 MHz). If the internal oscillator
block is supplying the system clock, changing the
states of these bits will have an immediate change on
the internal oscillator’s output.
• Secondary oscillators
• Internal oscillator block
The primary oscillators include the External Crystal
and Resonator modes, the External RC modes, the
External Clock modes and the internal oscillator block.
The particular mode is defined on POR by the contents
of Configuration Register 1H. The details of these
modes are covered earlier in this chapter.
The OSTS, IOFS and T1RUN bits indicate which clock
source is currently providing the system clock. The
OSTS indicates that the Oscillator Start-up Timer has
timed out and the primary clock is providing the system
clock in primary clock modes. The IOFS bit indicates
when the internal oscillator block has stabilized and is
providing the system clock in RC Clock modes. The
T1RUN bit (T1CON<6>) indicates when the Timer1
oscillator is providing the system clock in secondary
clock modes. If none of these bits are set, the INTRC is
providing the system clock, or the internal oscillator
block has just started and is not yet stable.
The secondary oscillators are those external sources
not connected to the OSC1 or OSC2 pins. These
sources may continue to operate even after the
controller is placed in a power managed mode.
PIC18F2X20/4X20 devices offer only the Timer1
oscillator as a secondary oscillator. This oscillator, in all
power managed modes, is often the time base for
functions such as a real-time clock.
Most often, a 32.768 kHz watch crystal is connected
between the RC0/T1OSO/T1CKI and RC1/T1OSI pins.
Like the LP mode oscillator circuit, loading capacitors
are also connected from each pin to ground.
The IDLEN bit controls the selective shutdown of the
controller’s CPU in power managed modes. The use of
these bits is discussed in more detail in Section 3.0
“Power Managed Modes”.
The Timer1 oscillator is discussed in greater detail in
Section 12.2 “Timer1 Oscillator”.
In addition to being a primary clock source, the internal
oscillator block is available as a power managed
mode clock source. The INTRC source is also used as
the clock source for several special features, such as
the WDT and Fail-Safe Clock Monitor.
Note 1: The Timer1 oscillator must be enabled to
select the secondary clock source. The
Timer1 oscillator is enabled by setting the
T1OSCEN bit in the Timer1 Control regis-
ter (T1CON<3>). If the Timer1 oscillator
is not enabled, then any attempt to set the
SCS0 bit will be ignored.
The clock sources for the PIC18F2X20/4X20 devices
are shown in Figure 2-8. See Section 12.0 “Timer1
Module” for further details of the Timer1 oscillator. See
Section 23.1 “Configuration Bits” for Configuration
register details.
2: It is recommended that the Timer1
oscillator be operating and stable before
executing the SLEEPinstruction or a very
long delay may occur while the Timer1
oscillator starts.
DS39599C-page 24
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
