PIC12F635ESLASHSS datasheet(31/196 Pages) MICROCHIP

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Preliminary

DS41232B-page 29

PIC12F635/PIC16F636/639

3.0

CLOCK SOURCES

3.1

Overview

The PIC12F635/PIC16F636/639 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

block

diagram

the

PIC12F635/PIC16F636/639 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

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 to the

internal oscillator.

The PIC12F635/PIC16F636/639 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 Resonator

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 (RC) with I/O

on RA4.

INTOSC – Internal oscillator with FOSC/4 output

on RA4 and I/O on RA5.

INTOSCIO – 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:

PIC12F635/PIC16F636/639 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(1)

8 MHz

LFINTOSC(2)

31 kHz

Note

HFINTOSC = High-Frequency Calibrated Internal Oscillator.

LFINTOSC = Low-Frequency Internal Oscillator is not calibrated.

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