[En-Nut-Discussion] SPI Bus

[Tim DeBaillie]

On Feb 21, 2009, at 5:48 AM, Bernd Walter <enut at cicely.de> wrote:

> On Sat, Feb 21, 2009 at 12:49:25AM -0300, Thiago A. Corrêa wrote:
>> Hi,
>>
>>   I'm now porting the SPI Bus, and as always, I'm taking the at91 as
>> example. Is there any reason why the at91 driver is not using the PCS
>> to let the chip auto manage the CSn line?
>
> The automangement often doesn't match with the slaves expectations.
> Some slaves require the CS to be asserted over a complete sequence,
> while the SPI controller deasserts it very quick.

You don't have the port setup correctly then. You can configure this.

>
> And sometimes people want more than 4 slaves.

The SAM can controll up to 15 devices with a simple 4-16 multiplexer.  
The SAM has that built in to the SPI controller. You just have to set  
it up and attach the multiplexer.

>
> I assume almost everyone is doing it manually.

Not true. We are using the PCS control on 4 different boards.

>
>
>>   Is the shadowing of the MR intentional? or just a consequence of
>> the CSR? I mean, can we have on the same bus peripherals with MODFDIS
>> and without?

You can. You just have to switch the MR before you transmit on a  
different device and switch it back when your done.  That's a little  
manual.

>>
>>
>>   I would appreciate some clarification on those. Thanks :)

Tim