Datasheet

PIC18F85J90 FAMILY
DS39770C-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<1:0> Configuration 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 4.0
“Power-Managed Modes”.
3.3.1.1 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 FOSC<1:0> (that is, one of the HS
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. This bit
determines whether the external or internal oscillator will
be the default clock source on subsequent device
Resets. By extension, it also has the effect of determin-
ing the clock source selected when SCS<1:0> are in
their Reset state (= 00). When FOSC2 = 1 (default), the
oscillator source defined by FOSC<1:0> is selected
whenever SCS<1:0> = 00. When FOSC2 = 0, the
internal oscillator block is selected whenever
SCS<1:2> = 00.
In those cases when the internal oscillator block 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 the setting of FOSC2, 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 or the internal
oscillator will have two bit setting options for the
possible values of SCS<1:0>, at any given time,
depending on the setting of FOSC2.
3.3.2 OSCILLATOR TRANSITIONS
PIC18F85J90 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.