[Gta04-owner] USB host vbus current draw

Paul Kocialkowski contact at paulk.fr
Tue Jun 17 19:35:32 CEST 2014

A few days ago, it was reported to me that some USB flash drive was not
working with the GTA04 USB host. The required current from the device
was 300mA, while I set the limit to 200mA a few months back.

So that got me wondering what the actual limitation of the USB host port
really is. To find out, I used a few resistors with very small values
and measured what current and voltage the USB host port provides.

* Off-load, vbus was measured at 5.12V
* With a 50.3 Ohm resistor (theoretical current of ~100mA with no
voltage drop), I get 4.9V and 95-100mA current draw.
* With a 15.7 Ohm resistor (theoretical current of ~326mA with no
voltage drop), I get 4.4V and 280mA current draw.
* With any value below 15.7 Ohm, vbus collapses to 0V and no current is
drawn at all until the device is rebooted.

Looking at these figures, it seems like 100mA is still reasonably-well
handled while anything beyond 300mA is impossible.
So I think it makes sense to keep the limit at 200mA (4.4V seems
unreasonably low, so I think even 250mA would be too much).

Paul Kocialkowski, Replicant developer

Replicant is a fully free Android distribution

Website: http://www.replicant.us/
Redmine: http://redmine.replicant.us/
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