PHSA/PHSB access from Spin

[Phil Pilgrim (PhiPi)]

Here's a program whose behavior perplexes me:




CON

_clkmode = xtal1 + pll16x
_xinfreq = 5_000_000

freq = 10000
advance = 6000
pina = 14
pinb = 15

PUB start

frqa := ((freq << 16) / clkfreq) << 16
ctra := constant(%00100 << 26) | pina
dira[pina]~~

frqb := ((freq << 16) / clkfreq) << 16
ctrb := constant(%00100 << 26) | pinb
dira[pinb]~~

repeat
phsb += advance
waitcnt(cnt + clkfreq * 2)




It starts two NCO clocks, both with the same frequency, with outputs on pins 14 and 15. In the repeat loop, phsb gets incremented by advance. But here's the weird thing: The amount by which the phase of pin 15, relative to that of pin 14, advances at each step (as seen on a scope) is independent of the value of advance, and equals about -3.7µs. What's up with that? Is Spin doing a read-modify-write on phsb, thereby reading phsb's shadow register? Well, I tested that theory by doing an explicit read, followed by a modify, then a write. 'Same results, with just a longer phase shift.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 1/8/2008 5:05:39 AM GMT

[OzStamp]

Hi Phil
have you tried
ctrb:= (constant (%00100 << 26)) | pinb

or····

ctrb := %00100<< 26 | pinb

just some guesses that I would try..


ron Mel oz

Post Edited (OzStamp) : 1/8/2008 5:36:38 AM GMT

[Phil Pilgrim (PhiPi)]

No, the counter configurations are okay, and they're outputting at the expected frequency. It's just the ability to adjust their relative phase on the fly, after configuration, that's got me bamboozled.

-Phil

[Tracy Allen]

Phil, was that indeed a phase retardation of -3.7 µs instead of advance?. hmmm

The value, advance=6000 is practically nothing in terms of phase, because phsb would be advancing by 524_288 (=2^19) every 12.5 ns. It should take a value of advance=155_189_248 to evoke a 3.7 µs phase advance. That is about 1/28 of the full cycle. Did you try numbers up in that range or higher, and still the same result?

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Tracy Allen
www.emesystems.com (http://www.emesystems.com)

[stevenmess2004]

It doesn't have to do with the spin execution timing does it?

Steven

[Phil Pilgrim (PhiPi)]

Tracy,

A phase retardation of x is just a phase advance of period - x, so that doesn't bother me, since I assume there's some Spin overhead that also gets added in.

I did try some numbers orders of magnitude bigger than 6000, but you're right: they weren't nearly big enough to make a noticeable difference. Now that I've tried some really, really big numbers, I'm seeing some effective differences. Thanks for making me revisit my assumptions! http://forums.parallax.com/images/smilies/smile.gif

-Phil

[deSilva]

The 3,7µs is exactly the time it takes SPIN between READING the PHS and RESTORING it (with practically the same value , as Tracy rightly remarked) http://forums.parallax.com/images/smilies/smile.gif

So you practically discard all the hardware induced PHS increments during that time, giving you the observed retardation. When beeing morte explicite that
PHSA += advance
this retardation will increase of course, as you already noticed.