DDS (generating sine waves) with onboard DAC using latest PIC 16F chips?

[HankMcSpank]

HenrikOlsson, kindly offered up this code he put together as a solution how to get an accumulator large than 16 bit (see this thread http://www.picbasic.co.uk/forum/show...943#post106943 ) ...

LSW  VAR WORD              ' Least significat word of accumulator
MSW  VAR WORD              ' Most significatn word of accumulator
 
ADDL VAR WORD              ' Least significant work of value to add
ADDH VAR WORD              ' Most significant word of value to add 
 
Out VAR BYTE                    ' This is the actual output from the lookup table
 
Temp VAR WORD
 
OverFlow VAR BIT             ' Gets set when 32bit accumulator overflows.
 
i VAR WORD
 
Init:
  LSW = 0 
  MSW = 0
  AddL = 500
  AddH = 2000   ' 50*256+500=768500
 
Pause 3000
 
Main:
For i = 1 to 1000
  OverFlow = 0
  TMR1H = 0
  TMR1L = 0
 
  Gosub Add
  Gosub GetValue
 
  HSEROUT["Count: ", DEC4 i, "  MSW: ", DEC5 MSW, "   LSW: ", DEC5 LSW, "   Overflow: ", BIN Overflow, "  Out: ", DEC Out, "  Ticks: ", DEC5 TMR1H*256+TMR1L, 13]
  Pause 5
NEXT
 
END
 
Add:
  T1CON = 1             ' This is just used to measure the execution time
 
  Temp = LSW            ' Remember least significant word
  LSW = LSW + ADDL      ' Add low word 
 
  If LSW < Temp Then ' Did we wrap around/overflow?
    MSW = MSW + 1       ' Increment high word
    If MSW = 0 Then OverFlow = 1  ' Did we overflow high word?
  ENDIF
 
  Temp = MSW            ' Remember high word
  MSW = MSW + ADDH      ' Add high word 
 
  If MSW < Temp Then ' Did we wrap around/overflow?
    OverFlow = 1     ' Set flag
  ENDIF
 
T1CON = 0
RETURN
 
GetValue:
Lookup MSW.HighBYTE, [$80,$83,$86,$89,$8C,$8F,$92,$95,$98,$9C,$9F,$A2,$A5,$A8,$AB,$AE,$B0,$B3,$B6,$B9,$BC,$BF,$C1,$C4,_
$C7,$C9,$CC,$CE,$D1,$D3,$D5,$D8,$DA,$DC,$DE,$E0,$E2,$E4,$E6,$E8,$EA,$EC,$ED,$EF,$F0,$F2,$F3,$F5,$F6,$F7,$F8,$F9,$FA,$FB,$FC,$FC, _
$FD,$FE,$FE,$FF,$FF,$FF,$FF,$FF,$FF,$FF,$FF,$FF,$FF,$FF,$FE,$FE,$FD,$FC,$FC,$FB,$FA,$F9,$F8,$F7,$F6,$F5,$F3,$F2,$F0,$EF,$ED,$EC, _
$EA,$E8,$E6,$E4,$E2,$E0,$DE,$DC,$DA,$D8,$D5,$D3,$D1,$CE,$CC,$C9,$C7,$C4,$C1,$BF,$BC,$B9,$B6,$B3,$B0,$AE,$AB,$A8,$A5,$A2,$9F,$9C, _
$98,$95,$92,$8F,$8C,$89,$86,$83,$7F,$7C,$79,$76,$73,$70,$6D,$6A,$67,$63,$60,$5D,$5A,$57,$54,$51,$4F,$4C,$49,$46,$43,$40,$3E,$3B, _
$38,$36,$33,$31,$2E,$2C,$2A,$27,$25,$23,$21,$1F,$1D,$1B,$19,$17,$15,$13,$12,$10,$0F,$0D,$0C,$0A,$09,$08,$07,$06,$05,$04,$03,$03, _
$02,$01,$01,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$01,$01,$02,$03,$03,$04,$05,$06,$07,$08,$09,$0A,$0C,$0D,$0F,$10,$12,$13, _
$15,$17,$19,$1B,$1D,$1F,$21,$23,$25,$27,$2A,$2C,$2E,$31,$33,$36,$38,$3B,$3E,$40,$43,$46,$49,$4C,$4F,$51,$54,$57,$5A,$5D,$60,$63, _
$67,$6A,$6D,$70,$73,$76,$79,$7C],Out
RETURN

Not sure I'll get time over this weekend, but intend trying it soon.

If it works (& I've no doubt it will!), then that just leaves on issue-ette - how to get a PIC to convert the required frequency (say arriving in from a serial port from a human or other pic), into the 'tuning word'.

required frequency/interrupt rate * accumulator size

so for a required frequency of 4971.21Hz involving a 32 bit accumulator & and say an interupt rate of 20,000Hz

(4971.21/20000) * 4294967296

the first part of the equation results in a decimal & then multiplying the decimal by a 32 bit number....woah! (can this even be done?!)