Datasheet

PIC12(L)F1501
DS41615A-page 48 Preliminary  2011 Microchip Technology Inc.
5.2.2.3 Internal Oscillator Frequency
Selection
The system clock speed can be selected via software
using the Internal Oscillator Frequency Select bits
IRCF<3:0> of the OSCCON register.
The outputs of the 16 MHz HFINTOSC postscaler and
the LFINTOSC connect to a multiplexer (see
Figure 5-1). The Internal Oscillator Frequency Select
bits IRCF<3:0> of the OSCCON register select the
frequency output of the internal oscillators. One of the
following frequencies can be selected via software:
•HFINTOSC
-16 MHz
-8 MHz
-4 MHz
-2 MHz
-1 MHz
- 500 kHz (default after Reset)
- 250 kHz
- 125 kHz
- 62.5 kHz
- 31.25 kHz
•LFINTOSC
-31 kHz
The IRCF<3:0> bits of the OSCCON register allow
duplicate selections for some frequencies. These dupli-
cate choices can offer system design trade-offs. Lower
power consumption can be obtained when changing
oscillator sources for a given frequency. Faster transi-
tion times can be obtained between frequency changes
that use the same oscillator source.
5.2.2.4 Internal Oscillator Clock Switch
Timing
When switching between the HFINTOSC and the
LFINTOSC, the new oscillator may already be shut
down to save power (see Figure 5-3). If this is the case,
there is a delay after the IRCF<3:0> bits of the
OSCCON register are modified before the frequency
selection takes place. The OSCSTAT register will
reflect the current active status of the HFINTOSC and
LFINTOSC oscillators. The sequence of a frequency
selection is as follows:
1. IRCF<3:0> bits of the OSCCON register are
modified.
2. If the new clock is shut down, a clock start-up
delay is started.
3. Clock switch circuitry waits for a falling edge of
the current clock.
4. Clock switch is complete.
See Figure 5-3 for more details.
If the internal oscillator speed is switched between two
clocks of the same source, there is no start-up delay
before the new frequency is selected.
Start-up delay specifications are located in the
oscillator tables of Section 27.0 “Electrical
Specifications”.
Note: Following any Reset, the IRCF<3:0> bits
of the OSCCON register are set to ‘0111’
and the frequency selection is set to
500 kHz. The user can modify the IRCF
bits to select a different frequency.