Datasheet
PIC12(L)F1501
DS40001615C-page 48 2011-2015 Microchip Technology Inc.
5.3 Clock Switching
The system clock source can be switched between
external and internal clock sources via software using
the System Clock Select (SCS) bits of the OSCCON
register. The following clock sources can be selected
using the SCS bits:
• Default system oscillator determined by FOSC
bits in Configuration Words
• Internal Oscillator Block (INTOSC)
5.3.1 SYSTEM CLOCK SELECT (SCS)
BITS
The System Clock Select (SCS) bits of the OSCCON
register selects the system clock source that is used for
the CPU and peripherals.
• When the SCS bits of the OSCCON register = 00,
the system clock source is determined by value of
the FOSC<1:0> bits in the Configuration Words.
• When the SCS bits of the OSCCON register = 01,
the system clock source is the secondary
oscillator.
• When the SCS bits of the OSCCON register = 1x,
the system clock source is chosen by the internal
oscillator frequency selected by the IRCF<3:0>
bits of the OSCCON register. After a Reset, the
SCS bits of the OSCCON register are always
cleared.
When switching between clock sources, a delay is
required to allow the new clock to stabilize. These oscil-
lator delays are shown in Table 5-2.
5.3.2 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.
TABLE 5-2: OSCILLATOR SWITCHING DELAYS
Switch From Switch To Oscillator Delay
Any clock source
LFINTOSC 1 cycle of each clock source
HFINTOSC 2 s (approx.)
ECH, ECM, ECL 2 cycles