dsPIC33FJ64GS406 datasheet(189/456 Pages) MICROCHIP

189 / 456 page

 2009-2012 Microchip Technology Inc.

DS70591E-page 189

dsPIC33FJ32GS406/606/608/610 and dsPIC33FJ64GS406/606/608/610

9.1

CPU Clocking System

The

dsPIC33FJ32GS406/606/608/610

and

dsPIC33FJ64GS406/606/608/610 devices provide six

system clock options:

• Fast RC (FRC) Oscillator

• FRC Oscillator with PLL

• Primary (XT, HS, or EC) Oscillator

• Primary Oscillator with PLL

• Low-Power RC (LPRC) Oscillator

• FRC Oscillator with Postscaler

• Secondary (LP) Oscillator

9.1.1

SYSTEM CLOCK SOURCES

The Fast RC (FRC) internal oscillator runs at a nominal

frequency of 7.37 MHz. User software can tune the

FRC frequency. User software can optionally specify a

factor (ranging from 1:2 to 1:256) by which the FRC

clock frequency is divided. This factor is selected using

the FRCDIV<2:0> (CLKDIV<10:8>) bits.

The primary oscillator can use one of the following as

its clock source:

• XT (Crystal): Crystals and ceramic resonators in

the range of 3 MHz to 10 MHz. The crystal is

connected to the OSC1 and OSC2 pins

• HS (High-Speed Crystal): Crystals in the range of

10 MHz to 50 MHz. The crystal is connected to

the OSC1 and OSC2 pins

• EC (External Clock): The external clock signal is

directly applied to the OSC1 pin

The secondary (LP) oscillator is designed for low power

and uses a 32.768 kHz crystal or ceramic resonator.

The LP oscillator uses the SOSCI and SOSCO pins.

The LPRC internal oscIllator runs at a nominal

frequency of 32.768 kHz. It is also used as a reference

clock by the Watchdog Timer (WDT) and Fail-Safe

Clock Monitor (FSCM).

The clock signals generated by the FRC and primary

oscillators can be optionally applied to an on-chip

Phase-Locked Loop (PLL) to provide a wide range of

output

frequencies

for

device

operation.

PLL

configuration is described in Section 9.1.3 “PLL

Configuration”.

The FRC frequency depends on the FRC accuracy

(see Table 27-20) and the value of the FRC Oscillator

Tuning register (see Register 9-4).

9.1.2

SYSTEM CLOCK SELECTION

The oscillator source 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 24.1 “Configuration Bits”

for further details.) The Initial Oscillator Selection Con-

figuration bits, FNOSC<2:0> (FOSCSEL<2:0>), and

the Primary Oscillator Mode Select Configuration bits,

POSCMD<1:0> (FOSC<1:0>), select the oscillator

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

primary oscillator is the default (unprogrammed)

selection.

The Configuration bits allow users to choose among

12 different clock modes, shown in Table 9-1.

The output of the oscillator (or the output of the PLL if

a PLL mode has been selected), FOSC, is divided by 2

to generate the device instruction clock (FCY) and the

peripheral clock time base (FP). FCY defines the

operating speed of the device and speeds up to

50 MIPS are supported by the device architecture.

Instruction execution speed or device operating

frequency, FCY, is given by Equation 9-1.

EQUATION 9-1:

DEVICE OPERATING

FREQUENCY

TABLE 9-1:

CONFIGURATION BIT VALUES FOR CLOCK SELECTION

FCY = FOSC/2

Oscillator Mode

Oscillator Source POSCMD<1:0> FNOSC<2:0> See Notes

Fast RC Oscillator with Divide-by-N (FRCDIVN)

Internal

111

1, 2

Fast RC Oscillator with Divide-by-16 (FRCDIV16)

Internal

110

Low-Power RC Oscillator (LPRC)

Internal

101

Secondary Oscillator (SOSC)

Secondary

100

—

Primary Oscillator (HS) with PLL (HSPLL)

Primary

011

—

Primary Oscillator (XT) with PLL (XTPLL)

Primary

011

—

Primary Oscillator (EC) with PLL (ECPLL)

Primary

011

Primary Oscillator (HS)

Primary

010

—

Primary Oscillator (XT)

Primary

010

—

Primary Oscillator (EC)

Primary

010

Fast RC Oscillator with PLL (FRCPLL)

Internal

001

Fast RC Oscillator (FRC)

Internal

000

Note 1: OSC2 pin function is determined by the OSCIOFNC Configuration bit.

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

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