external power with USB-host-mode

joerg joerg.twinklephone at gmx.de
Fri Apr 4 12:10:05 CEST 2008

We already have specified a Y-cable assembly to power GTA02 from 
USB-charger/other external source while in hostmode. It makes use of the fast 
charge detection logic, to switch to USB-host + fastcharge whenever 47kR is 
detected on ID pin of USB connector.

The standard way to detect connection of a hostmode-cable is short to ground 
for this pin.

In order to use the Y-cable *without* charger as a replacement for a standard 
hostmode-cable, to get usual powered host mode, i suggest a scheme in drivers 
logic like:

protocol logic:
ID detects <50kR    =>   USB host mode

ID detects 0R       =>   USB powered by GTA02
ID detects 45~50k   =>   1)check for external power, if found -> fast charge
 if no power after 30s:  2)power USB from GTA02, check for connected device 
 if no device found, disable USB power and start over with 1)

NB: when power was detected and fast charge mode is enabled, there will be no 
further checks like 2). Fast charge stays active until ID sees NC (cable 
disconnect) - this to avoid discharge when mains power of charger fails.
When USB is powered by GTA02 (device detected or 0R), there is no way to 
switch to fast charge, other than pull and reconnect cable.

This allows for the 
fast charger to immediately start charging on connect [as before],
usage of standard hostmode cable is as usual [as before],
Y-cable can be used in place of a standard hostmode adapter (<30 sec delay),
Y-cable can be used for external powered host mode (you have to apply power 
prior to connecting the device. This makes sense anyway)


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