PIC16F631/677/685/687/689/690
3.0
3.1
OSCILLATOR MODULE (WITH
FAIL-SAFE CLOCK MONITOR)
Overview
The Oscillator module can be configured in one of eight
clock modes.
1.
2.
3.
4.
5.
6.
7.
8.
EC – External clock with I/O on OSC2/CLKOUT.
LP – 32 kHz Low-Power Crystal mode.
XT – Medium Gain Crystal or Ceramic Resonator
Oscillator mode.
HS – High Gain Crystal or Ceramic Resonator
mode.
RC – External Resistor-Capacitor (RC) with
F
OSC
/4 output on OSC2/CLKOUT.
RCIO – External Resistor-Capacitor (RC) with
I/O on OSC2/CLKOUT.
INTOSC – Internal oscillator with F
OSC
/4 output
on OSC2 and I/O on OSC1/CLKIN.
INTOSCIO – Internal oscillator with I/O on
OSC1/CLKIN and OSC2/CLKOUT.
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 two
internal oscillators, with a choice of speeds selectable via
software. Additional clock features include:
• Selectable system clock source between external
or internal 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.
Clock Source modes are configured by the FOSC<2:0>
bits in the Configuration Word register (CONFIG). The
internal clock can be generated from two internal
oscillators. The HFINTOSC is a calibrated high-
frequency oscillator. The LFINTOSC is an uncalibrated
low-frequency oscillator.
FIGURE 3-1:
PIC
®
MCU CLOCK SOURCE BLOCK DIAGRAM
FOSC<2:0>
(Configuration Word Register)
SCS<0>
(OSCCON Register)
External Oscillator
OSC2
Sleep
OSC1
LP, XT, HS, RC, RCIO, EC
MUX
INTOSC
IRCF<2:0>
(OSCCON Register)
8 MHz
Internal Oscillator
4 MHz
2 MHz
Postscaler
101
100
500 kHz
250 kHz
125 kHz
LFINTOSC
31 kHz
31 kHz
011
010
001
000
MUX
1 MHz
HFINTOSC
8 MHz
111
110
System Clock
(CPU and Peripherals)
Power-up Timer (PWRT)
Watchdog Timer (WDT)
Fail-Safe Clock Monitor (FSCM)
©
2007 Microchip Technology Inc.
DS41262D-page 47
MICROCHIP [ MICROCHIP TECHNOLOGY ]
