Re: [PATCH 1/3] ARM: dts: omap5-board-common: enable rtc and charging of backup battery

[H. Nikolaus Schaller]

Am 13.01.2016 um 19:31 schrieb Nishanth Menon <nm@xxxxxx>:

> On 01/13/2016 12:08 PM, H. Nikolaus Schaller wrote:
> [...]
> 
>>> OK. So are we sure the TWL driver will never have to toggle this pin?
>> 
>> After studying the Palmas TRM it appears that this pin just should be "high"
>> to be able to write to RTC and some scratchpad register. If the Palmas OTP
>> is programmed to use gpio7 as msecure input.
> 
> Thanks for digging it up. we dont use the scratchpad, but in some cases
> where SoC cold reset is involved, those registers may store additional
> information.

I remember a similar thing from omap3-twl4030 where the boot source is stored
so that a warm reboot searches there. But I don#t know if the OMPAP5 Boot ROM
is using that.

> 
>> 
>> Since the scratchpad is not used we can permanently enable msecure. Which
>> means that we must somehow get the driving output to be "1".
>> 
>> This can be either done by
>> * a gpio with pull-up - switched to input mode as I proposed, or
> 
> I think you intended to suggest to do a mux to gpio with just pinmux
> pull?

Yes.

> The internal pull on padconf is very weak
> - for typical needs like
> these, it is rather suggested to stick with real GPIO drive to prevent
> conditions like noise interference(for example).


well, on OMAP5 pull up/down are astonishingly strong :)
100-250µA. Which translated roughly to 7 .. 18 kOhm @ 1.8V logic.
So a noise source must be coupled by an impedance in the 1 kOhm range.
This is quite rare. So I would not worry about that.

But if there is MUX_MODE1 for this purpose that works equally well, we
should use it instead of a gpin+pullup.

BR,
Nikolaus