[gta02-core] More audio considerations

[Werner Almesberger]

Christoph Mair wrote:
> Last monday I accidentally tried what happens when trying to boot with IO_3V3 
> shorted to GND (caused by a bad solder joint at the aux switch).
[...]
> 
> I could observe two possible behaviours before finding my mistake:
> a) Qi booted, loaded the kernel and after some time I got a kernel panic
> (AUX-LED flashing).

Hmm, that doesn't sound right. If IO_3V3 is really shorted to GND, the
anode of the AUX LED is grounded as well and the LED thus can't flash.

>> If they can't handle overcurrent, then we'd probably need an external
>> LDO with switch.
> I think they probably can until the device becomes too hot and turns off 
> everything.

Let me be more precise: there are a number of possible outcomes of an
overcurrent situation:

1) Permanent hardware damage: not very desirable.

2) LDO delivers the extra current until thermal shutdown stops it.

   This bears the risk of the overcurrent and/or the thermal shutdown
   compromising system operation. I.e., the phone may reset or shut
   down then inserting/removing the jack.
   
3) LDO delivers the nominal current and adjusts the voltage accordingly.

   In this case, the other supplies would be isolated from the shorted
   rail. (Except for the battery, but a few hundred mA for some fraction
   of a second are generally no problem here.) This would be nearly
   optimal. Unfortunately, there's nothing in the manual to suggest this
   is the case.

4) LDO shuts down its output and tries to restart later.

   This is what HCLDO does. However, the manual (repeatedly) only
   mentions this feature for HCLDO, not for any of the other LDOs, so
   it's unlikely that the other LDOs do this too. HCLDO supplies the
   SD/MMC slot, which has obvious overcurrent risks of its own, so we
   can't just swap LDOs to get HCLDO for audio.

I guess only an experiment will tell. I'm afraid the result will be
2).

- Werner