Datasheet

PIC18F87J90 FAMILY
DS39933D-page 38  2010 Microchip Technology Inc.
3.3.1 CLOCK SOURCE SELECTION
The System Clock Select bits, SCS<1:0>
(OSCCON<1:0>), select the clock source. The avail-
able clock sources are the primary clock defined by the
FOSC<2:0> Configuration bits, the secondary clock
(Timer1 oscillator) and the internal oscillator. The clock
source changes after one or more of the bits is 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
Secondary Clock modes. In power-managed modes,
only one of these bits will be set at any time. If neither
of these bits is 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 4.0
“Power-Managed Modes”.
3.3.1.1 System Clock Selection and Device
Resets
Since the SCS bits are cleared on all forms of Reset,
this means the primary oscillator defined by the
FOSC<2:0> Configuration bits is used as the primary
clock source on device Resets. This could either be the
internal oscillator block by itself or one of the other
primary clock source (HS, EC, HSPLL, ECPLL1/2 or
INTPLL1/2).
In those cases when the internal oscillator block, with-
out PLL, is the default clock on Reset, the Fast RC
oscillator (INTOSC) will be used as the device clock
source. It will initially start at 1 MHz; the postscaler
selection that corresponds to the Reset value of the
IRCF<2:0> bits (‘100’).
Regardless of which primary oscillator is selected,
INTRC will always be enabled on device power-up. It
serves as the clock source until the device has loaded
its configuration values from memory. It is at this point
that the FOSC Configuration bits are read and the
oscillator selection of the operational mode is made.
Note that either the primary clock source, or the internal
oscillator, will have two bit setting options for the possible
values of the SCS<1:0> bits at any given time.
3.3.2 OSCILLATOR TRANSITIONS
PIC18F87J90 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 4.1.2 “Entering 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.