PIC16F684-ESLASHST datasheet(21/164 Pages) MICROCHIP

21 / 164 page

 2004 Microchip Technology Inc.

Preliminary

DS41202C-page 19

PIC16F684

3.0

CLOCK SOURCES

3.1

Overview

The PIC16F684 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 PIC16F684 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.

The PIC16F684 can be configured in one of eight clock

modes.

EC – External clock with I/O on RA4.

LP – Low gain Crystal or Ceramic Resonator

Oscillator mode.

XT – Medium gain Crystal or Ceramic Resona-

tor Oscillator mode.

HS – High gain Crystal or Ceramic Resonator

mode.

RC – External Resistor-Capacitor (RC) with

FOSC/4 output on RA4.

RCIO – External Resistor-Capacitor with I/O on

RA4.

INTRC – Internal oscillator with FOSC/4 output

on RA4 and I/O on RA5.

INTRCIO – Internal oscillator with I/O on RA4

and RA5.

Clock source modes are configured by the FOSC<2:0>

bits

the

Configuration

Word

(see

Section 12.0 “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:

PIC16F684 CLOCK SOURCE BLOCK DIAGRAM

(CPU and Peripherals)

OSC1

OSC2

Sleep

External Oscillator

LP, XT, HS, RC, RCIO, EC

System Clock

8 MHz

4 MHz

2 MHz

1 MHz

500 kHz

125 kHz

250 kHz

IRCF<2:0>

111

110

101

100

011

010

001

000

31 kHz

Power-up Timer (PWRT)

FOSC<2:0>

(Configuration Word)

SCS

(OSCCON<0>)

Internal Oscillator

(OSCCON<6:4>)

Watchdog Timer (WDT)

Fail-Safe Clock Monitor (FSCM)

HFINTOSC

8 MHz

LFINTOSC

31 kHz

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