Re: Powering OMAP's pins

[Tomi Valkeinen <tomi.valkeinen@ti.com>]

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On Thu, 2012-09-27 at 11:51 -0700, Tony Lindgren wrote:
> * Tony Lindgren <tony@atomide.com> [120927 11:45]:
> > * Tomi Valkeinen <tomi.valkeinen@ti.com> [120927 00:20]:
> > 
> > > I could be mistaken how to HW works (but it does work like that for
> > > dss), but sounds to me that uart and gpio drivers (and perhaps some
> > > others, I didn't go through all the pins) need similar pin->regulator
> > > mapping as you suggested for omapdss.
> > 
> > Yes it seems that there are supply voltage regulator domains
> > that are specific to some subsystems. I wonder if these are needed
> > in all mux modes, or only when the pins are muxed for that particular
> > subsystem? It could be that the documentation is missing some
> > information here..
> > 
> > For example, what happens if you try to use some vdds_dsi powered
> > pin in GPIO mode without vdds_dsi?

I have not tested that.

> Seems like this may provide some clues from "3.6 Power-up and Power-down":
> 
>  "If the SDI, DSI, or CSI2 and CSIb interfaces are used in standard
>  LVCMOS mode (that is, GPIO mode) rather than PHY mode (that is, serial
>  differential mode), then vdds_sdi, vdds_dsi, vdds_csi2, and vdds_csib
>  may also be connected to vdds. Please, see the recommended SDI, DSI,
>  CSI2, and CSIb power supply noise of Table 3-5, Recommended Operating
>  Conditions."
> 
> So based on that it seems that tweaking of the regulators for these
> pins is only needed for DSS etc, not for GPIO or serial usage.

I don't read the above paragraph the same way.

What I think it means is that if, say, a board does not use DSI at all,
the vdds_dsi input pin in OMAP could be connected to vdds instead of the
normal DSI power from the TWL chip, thus making the OMAP's vdds_dsi
"always on". Which means that in some boards the vdds_dsi in TWL is not
needed if the pins are used.

However, that doesn't mean that the vdds_dsi input is always connected
to vdds if the pins are used for non-DSI uses. A board may well use only
some of the DSI pins for display, leaving the rest free for other uses.
E.g. on omap3 there are 6 DSI pins, and a display panel could well use
only 4 of them. If the 2 other pins are used as GPIOs, handling vdds_dsi
is still required when using those GPIOs.

So I think the above paragraph confirms that the power for the pins is
indeed required, and handling for the regulator is needed for GPIOs,
uarts, etc. also.

 Tomi


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