Re: [ECOS] SPI and 9 bit transfers (on AT91)

[Peter Niebert <peter@niebert.com>]

Just to be sure, this means that the count would be "shorts" rather  
than "bytes" in the case of transfers between 9 and 16 bits?
I will soon be able to post a patched version of the at91_spi driver  
that allows for different bit length transmits.

Cheers and thanks for eCos, again and again.

-- Peter


Le 27 mai 09 à 18:57, Bart Veer a écrit :

>>>>>> "Peter" == Peter Niebert <peter@niebert.com> writes:
>
>    Peter> SPI transfers can vary depending on the protocol between 8
>    Peter> and 16 bits. The generic SPI API suggests transfers in
>    Peter> multiples of 8bits though. How should this be interpreted
>    Peter> in cases of protocols with 9 or more bits? I suppose that
>    Peter> there is no other way than splitting such transfers to two
>    Peter> bytes. It could be acceptable to have this transparent in
>    Peter> the generic SPI API, but the byte order has to be clarified
>    Peter> somewhere.
>
> The current SPI already defines semantics for transfers of more than 8
> bits. See http://ecos.sourceware.org/docs-latest/ref/spi-api.html,
> section "Simple Transfers", and the description of the tx_data
> argument. Basically tx_data will be interpreted as an array of shorts
> instead of bytes, and only the bottom n bits of each short will be
> transferred.
>
> These semantics were appropriate for the ColdFire QSPI device, and the
> first eCos SPI driver targetted that device. It also seems like the
> simplest semantics from the perspective of application developers. I
> do not know how well it maps onto the AT91 hardware.
>
> Bart
>
> -- 
> Bart Veer                                   eCos Configuration  
> Architect
> eCosCentric Limited    The eCos experts      http://www.ecoscentric.com/
> Barnwell House, Barnwell Drive, Cambridge, UK.      Tel: +44 1223  
> 245571
> Registered in England and Wales: Reg No 4422071.


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