DVR21 datasheet(107/416 Pages) MICROCHIP

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DS30010118D-page 107

PIC24FJ256GA705 FAMILY

A recommended code sequence for a clock switch

includes the following:

Disable interrupts during the OSCCON register

unlock and write sequence.

Execute the unlock sequence for the OSCCON

high byte by writing 78h and 9Ah to

OSCCON<15:8> in two back-to-back instructions.

Write the new oscillator source to the NOSCx

bits in the instruction immediately following the

unlock sequence.

Execute the unlock sequence for the OSCCON

low byte by writing 46h and 57h to OSCCON<7:0>

in two back-to-back instructions.

Set the OSWEN bit in the instruction immediately

following the unlock sequence.

Continue to execute code that is not clock-sensitive

(optional).

Invoke an appropriate amount of software delay

(cycle counting) to allow the selected oscillator

and/or PLL to start and stabilize.

Check to see if OSWEN is ‘0’. If it is, the switch

was successful. If OSWEN is still set, then

check the LOCK bit to determine the cause of

the failure.

The core sequence for unlocking the OSCCON register

and initiating a clock switch is shown in Example 9-1.

EXAMPLE 9-1:

BASIC CODE SEQUENCE

FOR CLOCK SWITCHING

;Place the new oscillator selection in W0

;OSCCONH (high byte) Unlock Sequence

MOV

#OSCCONH, w1

MOV

#0x78, w2

MOV

#0x9A, w3

MOV.b

w2, [w1]

MOV.b

w3, [w1]

;Set new oscillator selection

MOV.b

WREG, OSCCONH

;OSCCONL (low byte) unlock sequence

MOV

#OSCCONL, w1

MOV

#0x46, w2

MOV

#0x57, w3

MOV.b

w2, [w1]

MOV.b

w3, [w1]

;Start oscillator switch operation

BSET

OSCCON, #0

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