
2010-2012 Microchip Technology Inc.
DS41412E-page 27
PIC18(L)F2X/4XK22
2.0
OSCILLATOR MODULE (WITH
FAIL-SAFE CLOCK MONITOR)
2.1
Overview
The oscillator module has a wide variety of clock
sources and selection features that allow it to be used
in a wide range of applications while maximizing perfor-
mance and minimizing power consumption.
Figure 2-1illustrates a block diagram of the oscillator module.
Clock sources can be configured from external
oscillators, quartz crystal resonators, ceramic resonators
and Resistor-Capacitor (RC) circuits. In addition, the
system clock source can be configured from one of three
internal oscillators, with a choice of speeds selectable via
software. Additional clock features include:
Selectable system clock source between external
or internal sources via software.
Two-Speed Start-up mode, which minimizes
latency between external oscillator start-up and
code execution.
Fail-Safe Clock Monitor (FSCM) designed to
detect a failure of the external clock source (LP,
XT, HS, EC or RC modes) and switch
automatically to the internal oscillator.
Oscillator Start-up Timer (OST) ensures stability
of crystal oscillator sources.
The primary clock module can be configured to provide
one of six clock sources as the primary clock.
1.
RC
External Resistor/Capacitor
2.
LP
Low-Power Crystal
3.
XT
Crystal/Resonator
4.
INTOSC
Internal Oscillator
5.
HS
High-Speed Crystal/Resonator
6.
EC
External Clock
The HS and EC oscillator circuits can be optimized for
power consumption and oscillator speed using settings
in FOSC<3:0>. Additional FOSC<3:0> selections
enable RA6 to be used as I/O or CLKO (FOSC/4) for
RC, EC and INTOSC Oscillator modes.
Primary
Clock
modes
are
selectable
by
the
FOSC<3:0> bits of the CONFIG1H Configuration
register. The primary clock operation is further defined
by these Configuration and register bits:
1.
PRICLKEN (CONFIG1H<5>)
2.
PRISD (OSCCON2<2>)
3.
PLLCFG (CONFIG1H<4>)
4.
PLLEN (OSCTUNE<6>)
5.
HFOFST (CONFIG3H<3>)
6.
IRCF<2:0> (OSCCON<6:4>)
7.
MFIOSEL (OSCCON2<4>)
8.
INTSRC (OSCTUNE<7>)
The HFINTOSC, MFINTOSC and LFINTOSC are
factory calibrated high, medium and low-frequency
oscillators, respectively, which are used as the internal
clock sources.