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2011-2015 Microchip Technology Inc.
DS40001609E-page 55
PIC16(L)F1508/9
5.3.5
CLOCK SWITCHING BEFORE
SLEEP
When clock switching from an old clock to a new clock
is requested just prior to entering Sleep mode, it is
necessary to confirm that the switch is complete before
the
SLEEP
instruction is executed. Failure to do so may
result in an incomplete switch and consequential loss
of the system clock altogether. Clock switching is
confirmed by monitoring the clock status bits in the
OSCSTAT register. Switch confirmation can be
accomplished by sensing that the ready bit for the new
clock is set or the ready bit for the old clock is cleared.
For example, when switching between the internal
oscillator with the PLL and the internal oscillator without
the PLL, monitor the PLLR bit. When PLLR is set, the
switch to 32 MHz operation is complete. Conversely,
when PPLR is cleared, the switch from 32 MHz
operation to the selected internal clock is complete.
5.4
Two-Speed Clock Start-up Mode
Two-Speed Start-up mode provides additional power
savings by minimizing the latency between external oscil-
lator start-up and code execution. In applications that
make heavy use of the Sleep mode, Two-Speed Start-up
will remove the external oscillator start-up time from the
time spent awake and can reduce the overall power con-
sumption of the device. This mode allows the application
to wake-up from Sleep, perform a few instructions using
the INTOSC internal oscillator block as the clock source
and go back to Sleep without waiting for the external
oscillator to become stable.
Two-Speed Start-up provides benefits when the oscillator
module is configured for LP, XT, or HS modes. The Oscil-
lator Start-up Timer (OST) is enabled for these modes
and must count 1024 oscillations before the oscillator
can be used as the system clock source.
If the oscillator module is configured for any mode
other than LP, XT or HS mode, then Two-Speed
Start-up is disabled. This is because the external clock
oscillator does not require any stabilization time after
POR or an exit from Sleep.
If the OST count reaches 1024 before the device enters
Sleep mode, the OSTS bit of the OSCSTAT register is
set and program execution switches to the external oscil-
lator. However, the system may never operate from the
external oscillator if the time spent awake is very short.
Note:
Executing a
SLEEP
instruction will abort
the oscillator start-up time and will cause
the OSTS bit of the OSCSTAT register to
remain clear.
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