USB current limit when phone off

Andy Green andy at
Thu Feb 5 14:42:01 CET 2009

Hash: SHA1

Somebody in the thread at some point said:

| I was surprised about the current used by my freerunner when off, from
| a dumb USB charger, that seems to have been more than 100mA.
| I have yet to do more tests because I am not really sure how much
| power was drawn...

In itself that makes sense... PMU charger is autonomous and only gets
reset in NOPOWER PMU state, ie, when no power is coming in and the
battery and backup battery are dead.

So if it was charging before you powered off, it will keep on charging
until the PMU charger decides the battery is full.

| My car charger has got a led that looks (I am not sure about how it is
| designed to work !) to be :
| - green when nothing is draining power (unplugged or my when my phone
is off).
| - amber when little current is drawn (my phone plugged in, on, and
| using about 100mA -so when nothing special done, and battery current
| reported by phone confirmed usb current was indeed about 100mA-)
| - red if more current is drawn (about 500mA, forced via
| usb_chage_limit_dangerous -battery charge current confirmed it was
| using about 500mA-)
| I wanted to charge my phone :
| - if plugged when off (ex. pulling bat to shutdown since plugging usb
| starts the phone...), led was green : no current. ok (that's sad but
| ok)

Ah well the USB power coming starting the phone is intentional, so it
can figure out what charging level to set by the CPU.

| - from Android (so was using 100mA from charger) : shutdown..... and
| led went red ! (sometimes turned to amber for about one minute, then
| back to red). Battery charge went from 13% to 36% in about 30 minutes,
| so looks like it was indeed more than 100mA (?)

Well it seems it is the charger pulling the current and not something else.

| => So it looks like shutting down from Android (where I did not force
| higher current), I could get in a forbidden state where USB current
| was more than 100mA ?!
| (well it was good for me since I wanted to charge my phone, but
| It was with Qi from a few days ago, and Michaels Trimarchi kernel v10
| (most recent one).
| I did not test yet with another distro.
| Of course I have not much real current measuments, and am not sure
| either that my charger have not some resistance that could have made
| the phone believe it was the wall charger... (but usually, if I do not
| force it, it never uses more than 100mA)

You can confirm what the phone thinks about the charging context with this

cat /sys/class/i2c-adapter/i2c-0/0-0073/pcf50633-mbc/usb_curlim

Unless we rewrite it when we're shutting down it should stick while 'off'.

- -Andy

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