PIC24FJ128GA008-I/PT【PDF 7/258 页】IC PIC MCU FLASH 128K 80TQFP

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PIC24FJ128GA010 FAMILY

DS39747F-page 104

2005-2012 Microchip Technology Inc.

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>

two

back-to-back

instructions.

Write new oscillator source to the NOSC control

bits in the instruction immediately following the

unlock sequence.

Execute the unlock sequence for the OSCCON

low

byte

writing

46h

and

57h

OSCCON<7:0> in two back-to-back instructions.

Set the OSWEN bit in the instruction immediately

following the unlock sequence.

6. 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 8-1.

EXAMPLE 8-1:

BASIC CODE SEQUENCE

FOR CLOCK SWITCHING

.global __reset

.include "p24fxxxx.inc"

.text

__reset:

;Place the new oscillator selection in W0

;OSCCONH (high byte) Unlock Sequence

DISI #18

PUSH

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

POP

.end

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