Datasheet

dsPIC30F6011/6012/6013/6014
DS70117F-page 150 © 2006 Microchip Technology Inc.
If the oscillator has a very slow start-up time coming out
of POR, BOR or Sleep, it is possible that the PWRT
timer will expire before the oscillator has started. In
such cases, the FSCM will be activated and the FSCM
will initiate a clock failure trap, and the COSC<1:0> bits
are loaded with FRC oscillator selection. This will effec-
tively shut off the original oscillator that was trying to
start.
The user may detect this situation and restart the
oscillator in the clock fail trap ISR.
Upon a clock failure detection, the FSCM module will
initiate a clock switch to the FRC oscillator as follows:
1. The COSC bits (OSCCON<13:12>) are loaded
with the FRC oscillator selection value.
2. CF bit is set (OSCCON<3>).
3. OSWEN control bit (OSCCON<0>) is cleared.
For the purpose of clock switching, the clock sources
are sectioned into four groups:
1. Primary
2. Secondary
3. Internal FRC
4. Internal LPRC
The user can switch between these functional groups
but cannot switch between options within a group. If the
primary group is selected, then the choice within the
group is always determined by the FPR<3:0>
Configuration bits.
The OSCCON register holds the control and status bits
related to clock switching.
• COSC<1:0>: Read-only status bits always reflect
the current oscillator group in effect.
• NOSC<1:0>: Control bits which are written to
indicate the new oscillator group of choice.
- On POR and BOR, COSC<1:0> and
NOSC<1:0> are both loaded with the
Configuration bit values FOS<1:0>.
• LOCK: The LOCK status bit indicates a PLL lock.
• CF: Read only status bit indicating if a clock fail
detect has occurred.
• OSWEN: Control bit changes from a ‘0’ to a ‘1’
when a clock transition sequence is initiated.
Clearing the OSWEN control bit will abort a clock
transition in progress (used for hang-up
situations).
If Configuration bits FCKSM<1:0> = 1x, then the clock
switching and Fail-Safe Clock monitoring functions are
disabled. This is the default Configuration bit setting.
If clock switching is disabled, then the FOS<1:0> and
FPR<3:0> bits directly control the oscillator selection
and the COSC<1:0> bits do not control the clock selec-
tion. However, these bits will reflect the clock source
selection.
20.2.8 PROTECTION AGAINST
ACCIDENTAL WRITES TO OSCCON
A write to the OSCCON register is intentionally made
difficult because it controls clock switching and clock
scaling.
To write to the OSCCON low byte, the following code
sequence must be executed without any other
instructions in between:
Byte write is allowed for one instruction cycle. Write the
desired value or use bit manipulation instruction.
To write to the OSCCON high byte, the following
instructions must be executed without any other
instructions in between:
Byte write is allowed for one instruction cycle. Write the
desired value or use bit manipulation instruction.
Note: The application should not attempt to
switch to a clock of frequency lower than
100 KHz when the fail-safe clock monitor is
enabled. If such clock switching is
performed, the device may generate an
oscillator fail trap and switch to the Fast RC
oscillator.
Byte Write “0x46” to OSCCON low
Byte Write “0x57” to OSCCON low
Byte Write “0x78” to OSCCON high
Byte Write “0x9A” to OSCCON high