[ltp] Thinkcentre M51 USB hotplug issue

[Dmitry E. Mikhailov]

On Monday 15 November 2010 17:27, Nate Bargmann wrote:
So what is different when Linux starts that
> it sees the devices compared to later during hotplug that it can't?  Is
> there some kind of hardware initialization at work here?

The difference is the USB part of the southbridge being unpowered when the 
device is inserted. <Captain Obvious? Sorry then>

MAY BE (it's no more than speculation) the tiny ESD damage while inserting the 
USB flash is doing it's job. There's one client's PC with the following 
protocol:
1)touch the metal part of USB connector on flash
2)touch the metal (conductive, not painted) part of PC case
3)slowly insert the flash

By doing 1st and 2nd you equalize your potential and the potential of a 
flashcard to the PC's. Stupid, yes, but it works. 

The most funny thing is the (partial) repair procedure.

If PC (Intel ICH5/ICH6 based) doesn't start at all, it's a good idea to check 
the resistance between USB data pins (2,3) and power pins (1,4). Usually one 
of data pins is shorted to one of the power pins (due to electrical 
insulation breakdown inside the southbridge). If you found one, get a (better 
current limited, variable voltage) power supply or a *BIG* capacitor (like 
10'000+ uF) and first try connecting 1.5V to the shorted pins. If it doesn't 
work, increase the voltage to (un-)reasonable limits.
The problem: southbridge's internal 3.3V power supply is shorted to ground 
*WITH* one of USB pins
The solution: BURN the short.
The 2nd problem: don't burn the whole southbridge :-)
The 2nd solution: first try a capacitor, monitor southbridge's temperature
After the short is cleared, the MoBo may be still dead. Check the southbridge 
3.3V power suply path. Something (MOSFETs usually) may be also burned. It's 
easily replaced.

FINALLY, THE RESULT: the PC with this (read, resurrected) motherboard is 
alive. The internal USB controller is seen by the OS, but nothing connected 
to it is seen. Just add an external USB controller and it's OK.

What it's all about:
You'd better not experiment with dying internal USB ports, use a PCI card.
You could buy a $1 USB extension cable, tape it to the front of your PC and be 
(more or less) happy. Probles with this extension cable is it's electrical 
resistance: voltage drop is big enough that USB 2.5" HDD's often do not spin 
up. Otherwise it's Ok. Or you could add up a powered USB hub - right on your 
desk.