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manually changing oscillator source on PIC24 [how-to]

 
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Oli
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manually changing oscillator source on PIC24 [how-to]
PostPosted: Fri Feb 12, 2010 5:33 am     Reply with quote
I needed to change the clock source and found the setup_oscillator function inadequate, especially if switching from a non-PLL mode to PLL mode. There is an unlocking sequence that must be followed first and executed in assembly.

The code below changes the NOSC 2:1 bits to 011 to select the oscillator source as primary, with PLL enabled.

Code:
   #asm
      //;Place the new oscillator selection in W0
      MOV.b #0x03, w0                                 // NOSC2:0 = 011 - primary osc with PLL
      //;OSCCONH (high byte) Unlock Sequence
      MOV #0x0743,w1
      MOV #0x78, w2
      MOV #0x9A, w3
      //DISI #0x5               // uncomment if you have interrupts enabled
      MOV.b w2, [w1]
      MOV.b w3, [w1]
      //;Set new oscillator selection
      MOV.b w0, [w1]
     
      mov #0x01, w0                                   // clock switching enabled
      //;OSCCONL (low byte) unlock sequence
      MOV #0x0742,w1
      MOV #0x46, w2
      MOV #0x57, w3
      //DISI #0x5                // uncomment if you have interrupts enabled
      MOV.b w2, [w1]
      MOV.b w3, [w1]
      //;Start oscillator switch operation
      MOV.b w0, [w1]
   #endasm


EDIT: The values 0x0742 and 0x0742 are the addresses for the OSCCON high and low bytes on the PIC24FJ256GB110

Last edited by Oli on Fri Feb 12, 2010 8:44 am; edited 2 times in total
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Joined: 27 Aug 2008
Posts: 2337
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PostPosted: Fri Feb 12, 2010 7:27 am     Reply with quote
Yes, curiously CCS PCD is using a non-operational unlocking sequence in some places. The below code (used in PCD #PIN_SELECT code) does not work, because W0 is loaded between both writes to the unlock register. In this case, the pin select register shall be proteceted, but they aren't.

Code:
00334:  MOV     #46,W0
00336:  MOV.B   W0L,742
00338:  MOV     #57,W0
0033A:  MOV.B   W0L,742
0033C:  BSET.B  742.6
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