concatenating 4 bytes into one pseudo 32 bit counter?

[HankMcSpank]

Wow Henrik...what can I say (thank heavens for folks like you & others on here)

I won't pretend to understand it all (once numbers start going above 256 values "woah, there goes scary stuff!").

re this bit...

Init:
  LSW = 0 
  MSW = 0
AddL = 500
AddH = 2000   ' 50*256+500=768500

Not understanding the bolded bits - can you please let me know what you're doing there?

I'm trying to see how/where I would enter what in DDS terms is called the tuning word (essentially the number that gets added to the accumulator each interrupt?

Also, I was hoping (eventually) that the tuning word would arrive serially (either being calculated manually or sent from another pic)....the maths are going to be a bit troubling...to glean the tuning word to set the required output frequency, it's

required frequency/interrupt rate * accumulator size

so for a wanted frequency of 4971Hz involving a 32 bit accumulator & and say an interupt rate of 20,000Hz

(4971/20000) * 4294967296

...that's gonna be a challenge in an 8 bit PIC?

[HenrikOlsson]

Hi,
You have the 32bit accumulator built up by the two WORDS, LSW and MSW. Then you have the 32bit value which gets added to the accumulator each iteration (each interrupt in your case) and that's the ADDL and ADDH words. Perhaps it's the invalid comment that's causing confusion....?

Lets take a 16bit WORD as an example.
myValue VAR WORD
myValue = 12345

Now, the WORD, which is two bytes holds the value 12345, if you'd look at the high byte of that word its value would be 48 and the value in the low byte would be 57. Why? Because 48*256+57=12345

Same thing with our accumulator and "adder value" but this time we're working with two WORDS.
ADDL = 500
ADDH = 2000

2000 * 65536 + 500 = 512500 which is the value getting added to the accumulator each time. In your example, 4971/20000*2^32 the, value to add the accumulator each time is 1067514121 or ADDH=16288, ADDL=64753. Or, which might be easier expressed in hex: 3FA0F909, see there's your two words, ADDH=$3FA0, ADDL=$F909

Let me know if you try it on some real hardware.

/Henrik.

[HankMcSpank]

Thanks Henrik....excellent stuff.

So, using your code where I have say a 20khz interrupt rate in place, would it be the extract below that goes into the actual 'accumulator addition & lookup' interrupt subroutine?

Add:                            ' start of interrupt interrupt routine?
  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
'
' 
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
 
@ INT_RETURN

[HankMcSpank]

Henrik, rather than have two threads running, if you don't mind I'll cut/paste your input over to the other thread http://www.picbasic.co.uk/forum/show...921#post106921 , which perhaps has a more appropriate title for the content we are now discussing (allowing others to find it easier)