Datasheet
2012 Microchip Technology Inc. DS30684A-page 31
PIC18(L)F2X/45K50
3.0 OSCILLATOR MODULE (WITH
FAIL-SAFE CLOCK MONITOR)
3.1 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 3-1
illustrates 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 (F
OSC/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. PCLKEN (CONFIG1H<5>)
2. PRISD (OSCCON2<2>)
3. CFGPLLEN (CONFIG1L<1>)
4. PLLEN (OSCCON2<4>)
5. IRCF<2:0> (OSCCON<6:4>)
6. INTSRC (OSCCON2<5>)
The HFINTOSC and INTRC are factory calibrated high
and low-frequency oscillators, respectively, which are
used as the internal clock sources.