PIC24FJ64GA datasheet(94/262 Pages) MICROCHIP

94 / 262 page

PIC24FJ64GA004 FAMILY

DS39881C-page 92

Preliminary

7.4

Clock Switching Operation

With few limitations, applications are free to switch

between any of the four clock sources (POSC, SOSC,

FRC and LPRC) under software control and at any

time. To limit the possible side effects that could result

from this flexibility, PIC24F devices have a safeguard

lock built into the switching process.

7.4.1

ENABLING CLOCK SWITCHING

To enable clock switching, the FCKSM1 Configuration

bit in Flash Configuration Word 2 must be programmed

to ‘0’. (Refer to Section 23.1 “Configuration Bits” for

further details.) If the FCKSM1 Configuration bit is

unprogrammed (‘1’), the clock switching function and

Fail-Safe Clock Monitor function are disabled. This is

the default setting.

The NOSCx control bits (OSCCON<10:8>) do not

control the clock selection when clock switching is dis-

abled. However, the COSCx bits (OSCCON<14:12>)

will reflect the clock source selected by the FNOSCx

Configuration bits.

The OSWEN control bit (OSCCON<0>) has no effect

when clock switching is disabled. It is held at ‘0’ at all

times.

OSCTUN: FRC Oscillator Tune Register

U-0

—

bit 15

bit 8

U-0

R/W-0

—

TUN5(1)

TUN4(1)

TUN3(1)

TUN2(1)

TUN1(1)

TUN0(1)

bit 7

bit 0

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

bit 15-6

Unimplemented: Read as ‘0’

bit 5-0

TUN5:TUN0: FRC Oscillator Tuning bits

011111 = Maximum frequency deviation

011110 =

•

000001 =

000000 = Center frequency, oscillator is running at factory calibrated frequency

111111 =

•

100001 =

100000 = Minimum frequency deviation

Note 1:

Increments or decrements of TUN5:TUN0 may not change the FRC frequency in equal steps over the

FRC tuning range, and may not be monotonic.

Note:

The primary oscillator mode has three

different submodes (XT, HS and EC)

which are determined by the POSCMDx

Configuration bits. While an application

can switch to and from primary oscillator

mode in software, it cannot switch

between the different primary submodes

without reprogramming the device.

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