linux-kernel - Re: [spi-devel-general] [PATCH v2] spi: Add support for the OpenCores SPI controller.

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Date:	Tue, 28 Apr 2009 13:54:11 -0700
From:	David Brownell <david-b@...bell.net>
To:	Florian Fainelli <florian@...nwrt.org>
Cc:	Thierry Reding <thierry.reding@...onic-design.de>,
	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [spi-devel-general] [PATCH v2] spi: Add support for the OpenCores SPI controller.

On Tuesday 28 April 2009, Florian Fainelli wrote:
> > > Is this the http://www.opencores.org/?do=project&who=spi core?
> >
> > Yes, it is.
> >
> > > Its summary says "Variable length of transfer word up to 32 bits";
> > > does that mean "configurable when core is synthesized" instead of
> > > truly "variable"?
> 
> This is indeed configured at synthesis time.

Now I'm confused again.  Thierry says (below) that the number of bits
can be set per-"transfer".

Now, I can easily understand that a *maximum* would be configured
at synthesis time ... if there's a 32-bit CPU or DMA engine, it'd
make very limited sense to interact using 128-bit I/O words.

Is there both a configurable maximum, *and* a word-size setting that
can be changed on the fly?  That's what I would expect; it's what
most other designs do.  The only time I've seen fixed "you must use
N-bit words" designs is on cost-eradicated 8-bit microcontrollers.

- Dave


> > That summary seems out-dated. The variable length of transfer word is
> > actually the maximum length of a single transfer and is 128 bits in the
> > latest version. So you get 4 registers, each 32 bits wide into which you
> > program the data you want to transfer. Then you set the number of bits of
> > that transfer so the core knows which registers and what bits of those
> > registers to shift out serially.



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