Re: [spi-devel-general] atmel_spi clock polarity

[David Brownell <david-b@pacbell.net>]

On Monday 18 February 2008, Atsushi Nemoto wrote:
> 	 IIRC the clock state follows
> CSRn.CPOL just before the real transfer.

No ... clock state should be valid *before* chipselect goes
active.  So I'm thinking the patch from Haavard is likely
the right change.


> Like this (previous transfer 
> was MODE 0, new transfer is MODE 3):
> 
>            T0                T1 T2
> 
> CS      ~~~|________________________________________________

So at T0, some chip is selected (and never deselected) ...

> 
> CLK     ______________________|~|___|~~~|___|~~~|___|~~~|___

... and at T1 CPOL is changed??  That's wrong.  There should
never be a partial clock period while a chipselect is active.
While it's inactive, sure -- no chip should care.


> 
> SO      ~~~~~~~~~~~~~~~~~~~~~~~~~~|___|~~~|___|~~~|___|~~~|_
>                                    MSB
> 
> T0-T1 was relatively longer then T1-T2.  I suppose T1 is not the
> point of updating MR register, but the point of starting DMA transfer.
>