General • Re: Using PIO to combine two 16-bit words into one 32-bit word

Code:

out ISR, 16out ISR, 16 ; I can't tell if this one works from the documentationmov X, ISR

The second "out ISR, 16" may not do what I'm looking for. It's hard to tell from the data sheet:

OUT ISR, count sets the input shift counter to count

This does not make it clear whether the ISR will shift up by count

It doesn't. The OUT instruction always writes 32 bits to the destination, with high-order bits filled with zeros. So in your proposed piece of code, the second OUT will overwrite the results of the first one (the first one might as well have been OUT NULL,15).

Only OUT instructions shift the OSR; only IN instructions shift the ISR.

An alternative is:

Code:

out ISR, 16in OSR, 16mov X, ISR

This will create the correct alignment in the ISR, but the data sheet does not make it clear whether "in OSR, count" modifies the OSR counter.

It doesn't. The counter only gets modified for instructions where it tells you so in the datasheet (so for IN, the input counter will be modified but nothing else). Of course the counter only matters if you are using autopull.

Unless I misunderstand, it appears that the fewest cycles for this operation is:

Code:

out ISR, 16out X, 16in X, 16mov X, ISR

Is there a faster way?

So you have the output FIFO configured for 16 bits and you are doing 16-bit writes from the CPU, wanting to reassemble them into 32-bits?

I can't see a faster way to do that (though that doesn't guarantee there isn't one I've missed). But my instinct is that you would be better off keeping the input as 32-bhit: it's much easier to split 32-bit input into 16-bit words (or throw away unwanted halfwords) than to glue the 16-bit values back together.

Statistics: Posted by arg001 — Fri Jan 19, 2024 2:21 pm

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