PIC24F04KA201 datasheet(84/224 Pages) MICROCHIP

84 / 224 page

PIC24F04KA201 FAMILY

DS39937B-page 82

Preliminary

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

The PIC24F04KA201 family devices consist of two

types of secondary oscillator:

- High-Power Secondary Oscillator

- Low-Power Secondary Oscillator

These can be selected by using the SOSCSEL

(FOSC<5>) bit.

• Fast Internal RC (FRC) Oscillator

- 8 MHz FRC Oscillator

- 500 kHz Lower Power FRC Oscillator

• Low-Power Internal RC (LPRC) Oscillator

The primary oscillator and 8 MHz 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 produce

the internal instruction cycle clock, FCY. In this docu-

ment, 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 (POR) 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 23.1

“Configuration Bits”

for further details). The Primary

Oscillator

Configuration

bits,

POSCMD<1:0>

(FOSC<1:0>), and the Initial Oscillator Select Configura-

tion bits, FNOSC<2:0> (FOSCSEL<2:0>), select the

oscillator source that is used at a POR. The FRC primary

oscillator with postscaler (FRCDIV) is the default (unpro-

grammed) selection. The secondary oscillator, or one of

the internal oscillators, may be chosen by programming

these bit locations. The EC mode frequency range

Configuration bits, POSCFREQ<1:0> (FOSC<4:3>),

optimize power consumption when running in EC

mode. The default configuration is “frequency range is

greater than 8 MHz”.

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 (FOSC<7:6>) are used

jointly to configure device clock switching and the

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

8 MHz FRC Oscillator with Postscaler

(FRCDIV)

Internal

111

1, 2

500 MHz FRC Oscillator with Postscaler

(LPFRCDIV)

Internal

110

Low-Power RC Oscillator (LPRC)

Internal

101

Secondary (Timer1) Oscillator (SOSC)

Secondary

100

Primary Oscillator (HS) with PLL Module

(HSPLL)

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

8 MHz FRC Oscillator with PLL Module

(FRCPLL)

Internal

001

8 MHz FRC Oscillator (FRC)

Internal

000

Note 1:

OSCO pin function is determined by the OSCIOFNC Configuration bit.

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

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