64GA106 datasheet(116/330 Pages) MICROCHIP

116 / 330 page

PIC24FJ256GA110 FAMILY

DS39905E-page 116

 2010 Microchip Technology Inc.

8.1

CPU Clocking Scheme

The system clock source can be provided by one of

four sources:

• Primary Oscillator (POSC) on the OSCI and

OSCO pins

• Secondary Oscillator (SOSC) on the SOSCI and

SOSCO pins

• Fast Internal RC (FRC) Oscillator

• Low-Power Internal RC (LPRC) Oscillator

The Primary Oscillator and FRC sources have the

option of using the internal 4x PLL. The frequency of

the FRC clock source can optionally be reduced by the

programmable clock divider. The selected clock source

generates the processor and peripheral clock sources.

The processor clock source is divided by two to pro-

duce the internal instruction cycle clock, FCY. In this

document, the instruction cycle clock is also denoted

by FOSC/2. The internal instruction cycle clock, FOSC/2,

can be provided on the OSCO I/O pin for some

operating modes of the Primary Oscillator.

8.2

Initial Configuration on POR

The oscillator source (and operating mode) that is used

at a device Power-on Reset event is selected using

Configuration bit settings. The oscillator Configuration

bit settings are located in the Configuration registers in

the program memory (refer to Section 25.1 “Configu-

ration

Bits”

for further details). The Primary

Oscillator

Configuration

bits,

POSCMD<1:0>

(Configuration Word 2<1:0>), and the Initial Oscillator

Select

Configuration

bits,

FNOSC<2:0>

(Configuration Word 2<10:8>), select the oscillator

source that is used at a Power-on Reset. The FRC

Primary Oscillator with Postscaler (FRCDIV) is the

default (unprogrammed) selection. The Secondary

Oscillator, or one of the internal oscillators, may be

chosen by programming these bit locations.

The Configuration bits allow users to choose between

the various clock modes, shown in Table 8-1.

8.2.1

CLOCK SWITCHING MODE

CONFIGURATION BITS

The

FCKSM

Configuration

bits

(Configuration

Word 2<7:6>) are used to jointly configure device clock

switching and the Fail-Safe Clock Monitor (FSCM).

Clock switching is enabled only when FCKSM1 is

programmed (‘0’). The FSCM is enabled only when

FCKSM<1:0> are both programmed (‘00’).

TABLE 8-1:

CONFIGURATION BIT VALUES FOR CLOCK SELECTION

Oscillator Mode

Oscillator Source

POSCMD<1:0>

FNOSC<2:0>

Note

Fast RC Oscillator with Postscaler

(FRCDIV)

Internal

111

1, 2

(Reserved)

Internal

110

Low-Power RC Oscillator (LPRC)

Internal

101

Secondary (Timer1) Oscillator

(SOSC)

Secondary

100

Primary Oscillator (XT) with PLL

Module (XTPLL)

Primary

011

Primary Oscillator (EC) with PLL

Module (ECPLL)

Primary

011

Primary Oscillator (HS)

Primary

010

Primary Oscillator (XT)

Primary

010

Primary Oscillator (EC)

Primary

010

Fast RC Oscillator with PLL Module

(FRCPLL)

Internal

001

Fast RC Oscillator (FRC)

Internal

000

Note 1:

OSCO pin function is determined by the OSCIOFCN Configuration bit.

This is the default oscillator mode for an unprogrammed (erased) device.

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