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

[HenrikOlsson]

Hi Hank,
Starting with your last question in the other thread. Yes, that's basically the code that would go into your ISR. But you can remove the TMR1 stuff and the whole overflow flag business, that was just a debug aid.

Here's it is repackaged:

Add:
    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
 
' Here you should move the value to where it should be, DAC, PWM, R2R now.
 
    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
    ENDIF
 
    Temp = MSW            ' Remember high word
    MSW = MSW + ADDH      ' Add high word 
RETURN

This thing, including the lookup table runs in about 75 cycles which I think is pretty good. It's not near the 35 cycles quoted in the doc you linked to but I think it's plenty for your needs.


I played around a bit and I'm not sure this is going to work for you (due to the high resoultion of 2 decimal places you need) but it might be worth a try.
Your formula: 4971.21 / 20000 * 2^32 = 1067559218 can be reorganised so it reads 4971.21 * (2^32/20000) or 4971.21 * 214748.36.

Since your maximum frequency is 5000Hz we can multiply that side of the equation by 10 and still have it fit in a WORD, when we do that we can divide the other side of the equation by 10 which makes IT TOO fit in a WORD so now we have 49712 * 21475 = 1067565200 which obviosuly won't fit.....

I then tried to extract the 32bit intermediate result of that 16 by 16 bit multiplication and it seems like it might work. If I'm not mistaken the high word is stored in system variable R0 and the low word in R2 so if we could extract those immediately after doing the multiplication we're pretty much good to go:

Frequency VAR WORD
Temp VAR WORD
 
Frequency = 49712   ' 4971.2Hz
Temp = Frequency * 21475    ' Produce the 32bit intermediate result
AddH = R0                           ' Get high word from R0
AddL = R2                           ' and low word from R2
Gosub PrintIt                       ' Display it
 
Pause 100
END
 
PrintIt:
  HSEROUT["Frequency: ", DEC Frequency/10, ".", DEC Frequency//10, "   AddValue: ", DEC AddH, " , ", DEC AddL,13] 
RETURN

The above shows AddH as 15699 and AddL as 60536. That's 15699 * 65536 + 60536 = 1028910200 when it "should" be 1028902365, not perfect but close.


Play with it, see if it works. If not you might want to look into Darrels N-Bit math routines, they should be pretty straight forward for this I think.

/Henrik.