[U-Boot] [PATCH] arm: at91sam9n12: change EBI IO to high drive mode

[Albert ARIBAUD]

Hi Michael,

On Wed, 17 Jul 2013 07:27:36 -0400, Michael Cashwell
<mboards at prograde.net> wrote:

> On Jul 17, 2013, at 6:42 AM, Albert ARIBAUD <albert.u.boot at aribaud.net> wrote:
> 
> > I understand the symptom. What I don't undestand is how come NAND
> > does not keep its data lines in high impedance when its chip select is
> > inactive, which it is when DDR is being accessed.
> 
> Chip selects prevent contention but they do not make the load vanish.
> A deselected chip is not electrically the same as a non-populated chip,
> especially at high frequencies.
> 
> Think of it another way. CMOS pins that are input-only are always high-z
> but they still must be counted as a load when adding up the fan out seen
> by the upstream output driver.

They must be counted as load alright, I understand this. But that
leaves / leads to some questions:

- how come the issue never appeared so far? If this is so basic a
  problem, I would have expected it to show up as soon as the SoC gets
  frequent use.

- doesn't the driving lines' fan-out take this load into account?
  Again, I would expect a device's drive to be strong enough to
  overcome a few Hi-Zs, since that's an expected type of load.

- why is the issue not symmetric? Techno's CMOS or CMOS-like, I guess,
  so there is no reason to pull up rather than down. Unless there are
  pulldowns in the NAND or elsewhere, but then it's not Hi-Z any more.

> -Mike

Amicalement,
-- 
Albert.