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

[HankMcSpank]

How is your filter constructed?

OP PIN -> 2.2K resistor------------------> scope
.....................................|
.................................01uf cap
.....................................|
...................................Gnd


ok, I've implemented an R2R ladder - much nicer....but not without issue.

The waveform is rather nice up to about 1.5khz, but then it starts degrading , this is 3.33khz..



ok, so that doesn't look too bad (but when you make it larger, it's steppy)

If I up the interrupt rate things get better, but I can't get the interrupt rate any faster than about 80khz, before I hit a wall.

Firstly with DTS_INT using 'pbp' ...woeful, can't get the interrupt rate above 20Khz

Next DTS_INT using 'asm' instead ...I can get the interrupt rate past 80khz before my scope measures a different output frequency to what it should be present.

there's almost nothing in my interrupt routine....

DDS:
ACCUMULATOR = ACCUMULATOR + tuning_word
Lookup Accumulator.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],PORTC
@ INT_RETURN

I'm now realising why everyone has assembly in their interrupts!