the root cause of flip-flop charging logic if you insert wall charger

[Andy Green]

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Somebody in the thread at some point said:
| Andy Green wrote:
|> | The root cause is the signal from VBUS rasies and drops rapidly when
|> | inserting a wall charger.
| [...]
|> It sounds like you were already in Linux doing this, so I wonder what is
|> the excuse of the adapter now to drop dead briefly on insertion?
|
| Is this VBUS coming out of the adapter (i.e., measured on the adapter's
| cable) or VBUS obtained by the system, i.e., subject to contact bounce
| and all that ?

This is looking at V+ at the USB socket, it is not physical bounce but
response of the adapter itself I think we find.

I took pictures of it when I did the destructive test, here is repeated
insertion of adapter.

It looks a mess because I was interested in finding overshoot at that
time and aggregated all the tests, but you can see clearly the deal.
Blue is USB V+ pin and yellow is VB_SYS IIRC.  After some high current
loading it down, the adapter backs off driving completely and then
retries after 100ms.  That was on A4 revision so it doesn't even have
VB_SYS cap at all IIRC.

It seems doubtful the 500mA USB limit that is meant to be active from
cold actually is.

- -Andy
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