PIC18F87J10 Datasheet, PDF (33/394 Pages) Microchip Technology – 64/80-Pin High-Performance, 1-Mbit Flash Microcontrollers with nanoWatt Technology

PIC18F87J10 FAMILY
2.6.1 OSCILLATOR CONTROL REGISTER
The OSCCON register (Register 2-2) controls several
aspects of the device clock’s operation, both in full
power operation and in power-managed modes.
The System Clock Select bits, SCS1:SCS0, select the
clock source. The available clock sources are the
primary clock (defined by the FOSC2:FOSC0 configu-
ration bits), the secondary clock (Timer1 oscillator) and
the internal oscillator. The clock source changes after
one or more of the bits are written to, following a brief
clock transition interval.
The OSTS (OSCCON<3>) and T1RUN (T1CON<6>)
bits indicate which clock source is currently providing
the device clock. The OSTS bit indicates that the
Oscillator Start-up Timer (OST) has timed out and the
primary clock is providing the device clock in primary
clock modes. The T1RUN bit indicates when the
Timer1 oscillator is providing the device clock in sec-
ondary clock modes. In power-managed modes, only
one of these bits will be set at any time. If neither of
these bits are set, the INTRC is providing the clock, or
the internal oscillator has just started and is not yet
stable.
The IDLEN bit determines if the device goes into Sleep
mode or one of the Idle modes when the SLEEP
instruction is executed.
The use of the flag and control bits in the OSCCON
register is discussed in more detail in Section 3.0
“Power-Managed Modes”.
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 select a
secondary clock source when executing a
SLEEP instruction will be ignored.
2: It is recommended that the Timer1
oscillator be operating and stable before
executing the SLEEP instruction or a very
long delay may occur while the Timer1
oscillator starts.
2.6.1.1
System Clock Selection and the
FOSC2 Configuration Bit
The SCS bits are cleared on all forms of Reset. In the
device’s default configuration, this means the primary
oscillator defined by FOSC1:FOSC0 (that is, one of the
HC or EC modes) is used as the primary clock source
on device Resets.
The default clock configuration on Reset can be
changed with the FOSC2 configuration bit. The effect of
this bit is to set the clock source selected when
SCS1:SCS0 = 00. When FOSC2 = 1 (default), the
oscillator source defined by FOSC1:FOSC0 is selected
whenever SCS1:SCS0 = 00. When FOSC2 = 0, the
INTRC oscillator is selected whenever
SCS1:SCS2 = 00. Because the SCS bits are cleared on
Reset, the FOSC2 setting also changes the default
oscillator mode on Reset.
Regardless of the setting of FOSC2, INTRC will always
be enabled on device power-up. It will serve as the
clock source until the device has loaded its configura-
tion values from memory. It is at this point that the
FOSC configuration bits are read and the oscillator
selection of operational mode is made.
Note that either the primary clock or the internal
oscillator will have two bit setting options, at any given
time, depending on the setting of FOSC2.
2.6.2 OSCILLATOR TRANSITIONS
PIC18F87J10 family devices contain circuitry to
prevent clock “glitches” when switching between clock
sources. A short pause in the device clock occurs dur-
ing the clock switch. The length of this pause is the sum
of two cycles of the old clock source and three to four
cycles of the new clock source. This formula assumes
that the new clock source is stable.
Clock transitions are discussed in greater detail in
Section 3.1.2 “Entering Power-Managed Modes”.
 2005 Microchip Technology Inc.
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