need help with direct digital synthesizer

IanM

I'm looking to cut just one instruction from the following code to get down to 7 so that it can run on 7 cogs and have a free cog for control. The code implements a pipelined DDS with an equivalent clock of 20MHz:

.ddsloop        movs tabi,tmp
                add   acc,phase
                mov   tmp,acc
                shr   tmp,#24 'get the high byte
.tabi           mov   outa,0-0
                mov   outa,#0 'allow other cogs access
                test  ina,mask wz 'time to exit?
            ifz jmp   #ddsloop

.mask           long  $10000000 'bit to exit on
.acc            long  0 'set up at init
.phase          long  0 'set up at init
.tmp            long  0

If I dispense with "test" as a means of exiting gracefully, then there are 7 instructions but i'll need to stop the cogs manually. That might not be a problem but I'm planning on driving the clock input of another prop and it might not like a big change in phase on it's clock input! So i'm looking for a gracefull way of exiting in 7 (or fewer) instructions.

Cheers, Ian

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Ian Mitchell
www.research.utas.edu.au

rokicki

Easiest way I see is to unroll the loop once, and then split the test of the pin and the djnz.

So:

.ddsloop movs tabi,tmp
add acc,phase
mov tmp,acc
shr tmp,#24 'get the high byte
.tabi mov outa,0-0
mov outa,#0 'allow other cogs access
test ina,mask wz 'time to exit?
movs tabi2,tmp
add acc,phase
mov tmp,acc
shr tmp,#24 'get the high byte
.tabi2 mov outa,0-0
mov outa,#0 'allow other cogs access
ifz jmp #ddsloop

IanM

How cool is that! Brilliant, thanks man!

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Ian Mitchell
www.research.utas.edu.au

Phil Pilgrim (PhiPi)

Ian,

How many bits of OUTA (and which ones) are you actually using for your synth output?

-Phil

IanM

I haven't decided yet. The sine lookup will be 256 entries out of cog memory - perhaps 10 bits to the port will suffice. But because it's a lookup, any pin combination will work. In fact, for one project i'll be using two sets in quadrature.

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Ian Mitchell
www.research.utas.edu.au

Phil Pilgrim (PhiPi)

Oh, okay. I was thinking that if you were using eight bits, there might be some clever way to press the video circuitry into service.

-Phil

nutson

Carefull, "test ina,mask" does not work, ina can only be used as a source. See the tricks and traps document. "test mask, ina" does work. I fell into this one myself....(only once)

Nico

IanM

yes, thanks for that

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Ian Mitchell
www.research.utas.edu.au