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

[HankMcSpank]

I think we're at crossed wires a little.

My first (present) plan of attack is to use the 5 bit internal DAC to get an anlogue output... (that's what my screenscrape was earlier - using the PIC's internal DAC to get a rough analogue output) clearly 32 steps is insufficent for sine waves, so I was trying to add an extra 2 bits by creatively using the PIC Fixed voltage reference (the PIC FVR output canfeed the DAC and the voltagae output can be set to 1.024V, 2.048V or 4.096V) - I might not be grasping a fundamental issue here, so I need to ponder this one until I have an "Oops, that was an embarrassing idea" moment!

If the above comes to nothing then I'll relent & use HPWM and a filter (I only had a brief go at this a year or so ago & didn't actually test how much the reactance of the cap in such an arrangement impacts the amplitude...over the range I'm interested (0-5khz) it mght not be that much. I'm of the opinion that I'll end up having to go this way...then all I have to crack is the accumulator problem (need more than 16 bits) & the little bit of floating point (to get the accumulator value needed for the required frequency)

Clearly if I end up going the R2R route, then I'll need to bring a 14 pin PIC into service (likely to be a 16f1824)

[HankMcSpank]

My first (present) plan of attack is to use the 5 bit internal DAC to get an anlogue output... (that's what my screenscrape was earlier - using the PIC's internal DAC to get a rough analogue output) clearly 32 steps is insufficent for sine waves, so I was trying to add an extra 2 bits by creatively using the PIC Fixed voltage reference (the PIC FVR output canfeed the DAC and the voltagae output can be set to 1.024V, 2.048V or 4.096V) - I might not be grasping a fundamental issue here, so I need to ponder this one until I have an "Oops, that was an embarrassing idea" moment!

Well, I reckon this one is doomed ...I've only just realised for my plan to work, I would need to be able to shift the 'negative ref' of the DAC to the FVR module output (I'd then get 32 steps between gnd & FVR ref, then another 32 steps between FVR & Vref) - the 16f1828 DAC control register doesn't allow this...hey ho.

Ok, so HPWM and a filter it is then...at least I'll see what impact there is on amplitude as the frequency rises!

[cncmachineguy]

Ok, so I am now up to speed with you who is up to speed with me. We can now plod along blindly together - LOL. I was thinking of doing this all with software so as not to be dependent on HPWM. Chasing the single low cost PIC for 1 purpose, I was assuming HPWM may not be available.

Quick question, what is the fastest time needed? ie, 5,000hz has a time of .2mS, but 5,000.01 has a time of .199999mS. so you want .1uS, .01uS,1uS,bla bla bla. on the other hand 100hz is 10mS while 100.01Hz is 9.9999mS.

As you can see, I am having conceptual trouble here with the .01Hz spec.

[HankMcSpank]

Ok, so I am now up to speed with you who is up to speed with me. We can now plod along blindly together - LOL. I was thinking of doing this all with software so as not to be dependent on HPWM. Chasing the single low cost PIC for 1 purpose, I was assuming HPWM may not be available.

Quick question, what is the fastest time needed? ie, 5,000hz has a time of .2mS, but 5,000.01 has a time of .199999mS. so you want .1uS, .01uS,1uS,bla bla bla. on the other hand 100hz is 10mS while 100.01Hz is 9.9999mS.

As you can see, I am having conceptual trouble here with the .01Hz spec.

Maybe I'm too ambitious with the .01Hz accuracy, maybe .1hz is more achievable (initially)....as it goes, I actually only need that accuracy at the lower end of the 60hz-5,000Hz frequency range (where obviously 0.1Hz error of say 60hz is a whole lot more in percentage terms than .1Hz for 5khz) but with DDS you have to decide on the acceptable resolution then be stuck with it across the frequency band. so yes, say 50.01Hz, 50.02Hz, 50.3Hz thru 5,000Hz is what I hope to achieve here.

I've read that DDS 'interrupt rate' should be at least twice the targetted/required highest freqency, which since I want 5Khz, should mean a minimum interrupt of 10khz...but I thought 20Khz might be better. So with a 16 bit accumulator, we get....

audio ouput frequency granularity = 20,000/65536 or 0.31hz (which is not enough) ...even dropping the interrupt rate to 10khz means granularity of 0.15Hz)

That said, for proof of concept, maybe I just go with that initially ....ie 10,000 interrupts per second and a simple 16 bit accumulator - once that's been proven, then we can stride out a bit!

[cncmachineguy]

hank, your PM is full

[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?!)

[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.