Re: [spi-devel-general] atmel_spi clock polarity
From: David Brownell
Date: Mon Feb 18 2008 - 15:16:44 EST
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.
>
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