DI25 datasheet(98/416 Pages) MICROCHIP

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PIC24FJ256GA705 FAMILY

DS30010118D-page 98

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9.1

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 PLL block, which can

generate a 4x, 6x or 8x PLL clock. If the PLL is used,

the PLL clocks can then be postscaled, if necessary,

and used as the system clock. Refer to Section 9.5

“Oscillator Modes” for additional information. The

internal FRC provides an 8 MHz clock source.

Each clock source (PRIPLL, FRCPLL, PRI, FRC,

LPRC and SOSC) can be used as an input to an

additional divider, which can then be used to produce a

divided clock source for use as a system clock

(OSCFDIV).

The selected clock source generates the processor

and peripheral clock sources. The processor clock

source is divided by two to produce the internal instruc-

tion 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.

9.2

Initial Configuration on POR

The oscillator source (and operating mode) that is used

at a device Power-on Reset event is selected using Con-

figuration bit settings. The Oscillator Configuration bit

settings are located in the Configuration registers in the

program memory (refer to Section 29.1 “Configuration

Bits” for further details). The Primary Oscillator

Configuration bits, POSCMD<1:0> (FOSC<1:0>), and

the Oscillator Select Configuration bits, FNOSC<2:0>

(FOSCSEL<2:0>), select the oscillator source that is

used at a Power-on Reset. The OSCFDIV clock source

is the default (unprogrammed) selection; the default input

source to the OSCFDIV divider is the FRC clock source.

Other oscillators may be chosen by programming these

bit locations.

The Configuration bits allow users to choose between

the various Clock modes shown in Table 9-1.

9.2.1

CLOCK SWITCHING MODE

CONFIGURATION BITS

The FCKSM<1:0> Configuration bits (FOSC<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’).

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