PIC16F688 Datasheet, PDF (23/174 Pages) Microchip Technology – 14-Pin Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

3.0 CLOCK SOURCES
3.1 Overview
The PIC16F688 has a wide variety of clock sources
and selection features to allow it to be used in a wide
range of applications while maximizing performance
and minimizing power consumption. Figure 3-1
illustrates a block diagram of the PIC16F688 clock
sources.
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 Clock Start-up mode, which mini-
mizes 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 to the
Internal Oscillator.
PIC16F688
The PIC16F688 can be configured in one of eight clock
modes.
1. EC – External clock with I/O on RA4.
2. LP – Low gain Crystal or Ceramic Resonator
Oscillator mode.
3. XT – Medium gain Crystal or Ceramic Resona-
tor Oscillator mode.
4. HS – High gain Crystal or Ceramic Resonator
mode.
5. RC – External Resistor-Capacitor (RC) with
FOSC/4 output on RA4.
6. RCIO – External Resistor-Capacitor with I/O on
RA4.
7. INTRC – Internal oscillator with FOSC/4 output
on RA4 and I/O on RA5.
8. INTRCIO – Internal oscillator with I/O on RA4
and RA5.
Clock source modes are configured by the FOSC<2:0>
bits in the Configuration Word register (see
Section 11.0 “Special Features Of The CPU”). The
internal clock can be generated by two oscillators. The
HFINTOSC is a high-frequency calibrated oscillator.
The LFINTOSC is a low-frequency uncalibrated
oscillator.
FIGURE 3-1:
PIC16F688 CLOCK SOURCE BLOCK DIAGRAM
OSC2
OSC1
External Oscillator
Sleep
Internal Oscillator
HFINTOSC
8 MHz
LFINTOSC
31 kHz
FOSC<2:0>
(Configuration Word)
SCS
(OSCCON<0>)
LP, XT, HS, RC, RCIO, EC
IRCF<2:0>
(OSCCON<6:4>)
8 MHz
111
4 MHz
110
2 MHz
101
1 MHz
100
500 kHz
011
250 kHz
010
125 kHz
001
31 kHz ₀₀₀
System Clock
(CPU and Peripherals)
Power-up Timer (PWRT)
Watchdog Timer (WDT)
Fail-Safe Clock Monitor (FSCM)
 2004 Microchip Technology Inc.
Preliminary
DS41203B-page 21