Man-wai Chang said:
Is it likely for a voltage/current regulator on a motherboard to report
a false over-current status?
I am suspecting that my older motherboard Asus M3N78-PRO was exactly
reporting the wrong over-current status...
On a laptop, an 8 pin DIP chip measures the current. If the port
draws well in excess of 500mA per port, a MOSFET inside the chip,
turns off the port. This allows the laptop to protect the port,
without using fuses at all. This method is precise, because it
doesn't rely on the variable properties of fuse devices.
Your Asus motherboard, uses a Polyfuse per two ports. The voltage
level on the +5V rail as it feeds the USB port, also feeds the OC#
signal on the Southbridge. If the fuse opens, the level drops
to zero, and that logic zero is detected by the Southbridge
as a "USB overcurrent". The OS then responds with a dialog box.
+5V_motherboard ----- Polyfuse ---+--- +5V_to_USB_connectors ---- Upper
| ---- Lower
|
(To OC#, per port <---------------+
on the Southbridge)
If everything was working properly, seeing that status message
tells you a USB peripheral used more than 1.1 amps from +5V. The
green colored fuse near the USB port has a legend printed on it
that says "110" which stands for 1.1 amps.
There is another way an overcurrent indication can happen. Some
bad BIOS code exists on some motherboards, such that if you have
4GB of RAM installed, the BIOS does a poor job of defining
a memory map, and the overcurrent register and interface
appear to overlap with something else. This results in
erroneous overcurrent indications.
You check for that one, by browsing vip.asus.com forums
for your motherboard model, and look to see if anyone
reports spurious "USB overcurrent". Since there are
many threads here, I'll leave it to you to check this
http://vip.asus.com/forum/topic.aspx?board_id=1&model=M3N78+Pro&SLanguage=en-us
*******
If you cannot find any threads on "USB overcurrent" in that forum,
then the fuse must really be opening. You will need to
monitor the voltage on the port side of the Polyfuse, to
detect that is really happening. My favorite way of doing
this, would be with a LED and a resistor, and I know
you like projects like that
You are the king of
resistors, after all. The LED would glow, for as long
as the port has power.
+5V_motherboard ----- Polyfuse ---+--- +5V_to_USB_connector
|
| (+) (-)
(To OC#, per port <---------------+--- 330 ohm ---- LED --- ground
on the Southbridge)
Paul
