Datasheet

PIC18F46J11 FAMILY
DS39932D-page 42  2011 Microchip Technology Inc.
3.3 Clock Sources and Oscillator
Switching
Like previous PIC18 enhanced devices, the
PIC18F46J11 family includes a feature that allows the
device clock source to be switched from the main
oscillator to an alternate, low-frequency clock source.
PIC18F46J11 family devices offer two alternate clock
sources. When an alternate clock source is enabled,
the various power-managed operating modes are
available.
Essentially, there are three clock sources for these
devices:
• Primary Oscillators
• Secondary Oscillators
• Internal Oscillator Block
The Primary Oscillators include the External Crystal
and Resonator modes, the External Clock modes and
the internal oscillator block. The particular mode is
defined by the FOSC<2:0> Configuration bits. The
details of these modes are covered earlier in this
chapter.
The Secondary Oscillators are external sources that
are not connected to the OSC1 or OSC2 pins. These
sources may continue to operate even after the
controller is placed in a power-managed mode.
PIC18F46J11 family devices offer the Timer1 oscillator
as a secondary oscillator. This oscillator, in all
power-managed modes, is often the time base for
functions such as a Real-Time Clock (RTC). Most often,
a 32.768 kHz watch crystal is connected between the
RC0/T1OSO/T1CKI/RP11 and RC1/T1OSI/RP12 pins.
Like the HS Oscillator mode circuits, loading capacitors
are also connected from each pin to ground. The Timer1
oscillator is discussed in more detail in Section 13.5
“Timer1 Oscillator”.
In addition to being a primary clock source, the
postscaled internal clock is available as a
power-managed mode clock source. The INTRC
source is also used as the clock source for several
special features, such as the WDT and Fail-Safe Clock
Monitor (FSCM).
REGISTER 3-1: OSCTUNE: OSCILLATOR TUNING REGISTER (ACCESS F9Bh)
R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
INTSRC PLLEN TUN5 TUN4 TUN3 TUN2 TUN1 TUN0
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7 INTSRC: Internal Oscillator Low-Frequency Source Select bit
1 = 31.25 kHz device clock derived from 8 MHz INTOSC source (divide-by-256 enabled)
0 = 31 kHz device clock derived directly from INTRC internal oscillator
bit 6
PLLEN: Frequency Multiplier Enable bit
1 = PLL enabled
0 = PLL disabled
bit 5-0 TUN<5:0>: Frequency Tuning bits
011111 = Maximum frequency
011110
•
•
•
000001
000000 = Center frequency; oscillator module is running at the calibrated frequency
111111
•
•
•
100000 = Minimum frequency