PIC18F4321 Datasheet, PDF (28/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology

PIC18F4321 FAMILY
2.6 Internal Oscillator Block
The PIC18F4321 family of devices includes an internal
oscillator block which generates two different clock
signals; either can be used as the microcontroller’s
clock source. This may eliminate the need for external
oscillator circuits on the OSC1 and/or OSC2 pins.
The main output (INTOSC) is an 8 MHz clock source,
which can be used to directly drive the device clock. It
also drives a postscaler, which can provide a range of
clock frequencies from 31 kHz to 4 MHz. The INTOSC
output is enabled when a clock frequency from 125 kHz
to 8 MHz is selected. The INTOSC output can also be
enabled when 31 kHz is selected, depending on the
INTSRC bit (OSCTUNE<7>).
The other clock source is the internal RC oscillator
(INTRC), which provides a nominal 31 kHz output.
INTRC is enabled if it is selected as the device clock
source; it is also enabled automatically when any of the
following are enabled:
• Power-up Timer
• Fail-Safe Clock Monitor
• Watchdog Timer
• Two-Speed Start-up
These features are discussed in greater detail in
Section 23.0 “Special Features of the CPU”.
The clock source frequency (INTOSC direct, INTRC
direct or INTOSC postscaler) is selected by configuring
the IRCF bits of the OSCCON register (page 31).
2.6.1 INTIO MODES
Using the internal oscillator as the clock source elimi-
nates the need for up to two external oscillator pins,
which can then be used for digital I/O. Two distinct
configurations are available:
• In INTIO1 mode, the OSC2 pin outputs FOSC/4,
while OSC1 functions as RA7 (see Figure 2-8) for
digital input and output.
• In INTIO2 mode, OSC1 functions as RA7 and
OSC2 functions as RA6 (see Figure 2-9), both for
digital input and output.
FIGURE 2-8: INTIO1 OSCILLATOR MODE
RA7
FOSC/4
I/O (OSC1)
OSC2
PIC18FXXXX
FIGURE 2-9: INTIO2 OSCILLATOR MODE
RA7
I/O (OSC1)
PIC18FXXXX
RA6
I/O (OSC2)
2.6.2 INTOSC OUTPUT FREQUENCY
The internal oscillator block is calibrated at the factory
to produce an INTOSC output frequency of 8 MHz.
The INTRC oscillator operates independently of the
INTOSC source. Any changes in INTOSC across
voltage and temperature are not necessarily reflected
by changes in INTRC or vice versa.
2.6.3 OSCTUNE REGISTER
The INTOSC output has been calibrated at the
factory but can be adjusted in the user’s application.
This is done by writing to TUN4:TUN0
(OSCTUNE<4:0>) in the OSCTUNE register
(Register 2-1).
When the OSCTUNE register is modified, the INTOSC
frequency will begin shifting to the new frequency. The
INTOSC clock will stabilize within 1 ms. Code execu-
tion continues during this shift. There is no indication
that the shift has occurred. The INTRC is not affected
by OSCTUNE.
The OSCTUNE register also implements the INTSRC
(OSCTUNE<7>) and PLLEN (OSCTUNE<6>) bits,
which control certain features of the internal oscillator
block. The INTSRC bit allows users to select which
internal oscillator provides the clock source when the
31 kHz frequency option is selected. This is covered in
greater detail in Section 2.7.1 “Oscillator Control
Register”.
The PLLEN bit controls the operation of the Phase
Locked Loop (PLL) in Internal Oscillator modes (see
Figure 2-10).
FIGURE 2-10:
INTOSC AND PLL BLOCK
DIAGRAM
8 or 4 MHz
PLLEN
(OSCTUNE<6>)
INTOSC
FIN
FOUT
Phase
Comparator
Loop
Filter
CLKO
OSC2
÷4
RA6
VCO
SYSCLK
DS39689E-page 26
Preliminary
© 2007 Microchip Technology Inc.