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

[Michael Cashwell]

On Jul 17, 2013, at 9:05 AM, Albert ARIBAUD <albert.u.boot at aribaud.net> wrote:

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

Yes, agreed. It seems something changed. That needs to be understood.

> - 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.

Setting the drive strength in the CPU is how fan out is accounted for.
(And I've seen similar settings in DDR chip so their drive is right
going the other way.)

In cases like this where it's adjustable the goal is to use the lowest
drive level that covers the fan out. Too high wastes power and generates
heat. Too low is unreliable.

> - 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.


I'm not sure what symmetry you mean.

Very weak pulls are normal to avoid floating lines which can cause
damage to CMOS inputs. But these are in the 100KΩ range. That shouldn't
count more than one load. Strong pulls built into devices are a problem
when connected in parallel.

But again, it all does beg the question of what changed.

-Mike