Re: USB device shuts down PC
- From: "Ian D" <taurus@xxxxxxxxxxx>
- Date: Sun, 17 May 2009 22:12:46 -0400
"Paul" <nospam@xxxxxxxxxx> wrote in message
news:guqcvi$ni0$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Neil wrote:
Hi,
I have a PC running Windows XP home. I have both front & rear USB ports.
If I plug a printer into any of the USB ports, the PC shuts down with out
warning.
If I plug a USB memory stick into any of the ports, the same thing
happens.
I have a USB extension lead, if I plug the extension lead into any USB
port and then plug the memory stick into the extension it works OK.
I have recently installed a PCI USB card, the computer still shuts down
when devices are plugged in.
Any suggestions?
Thank you.
Modern computers sometimes choose to power the motherboard USB
connectors from +5VSB. If you read the label on the side of the
supply, the 5VSB output is rated for about 2 amps. That is only
enough current, to power four USB devices loading to the max
of 500mA each. (And there are other loads in the design, on the
+5VSB rail, so the 2 amps cannot be used entirely for USB.)
The PCI USB card, could power its ports from the regular +5V port.
If so, its ports may behave differently.
You may get some idea, which rail is used for the ports, if a
USB device with a LED on it, continues to operate the LED when
the computer is in sleep state. (A USB LED reading lamp is another
device you could use to check it.)
If +5VSB is momentarily overloaded, that can be enough to cause
the PS_ON# control signal to be deasserted, which in turn causes
the power supply to go off.
A second reason for +5VSB to glitch, is if the proper precautions
are not observed in the design. Intel recommends that a bulk capacitor
be placed near the USB port. The purpose of that capacitor, is to
provide current, to compensate for the "inrush" current that flows
into a USB device freshly plugged in. The USB device itself, should
not use gobs of its own bulk capacitance, as that makes inrush at
its input terminals, worse. In situations where the power supply
is otherwise healthy, and yet there is still a problem when a
USB device is plugged in, it could be a transient problem caused
by inrush at the device, or insufficient bulk capacitance at the
connector.
There are limits as to how much bulk capacitance can be used
in any design, because the ATX power supply likely has a
"stability limit" with respect to total capacitance on the
output. I mention that, because there are people out there who'll
reach for the biggest capacitor they've got, and slap it into
their PC. The following table is from a formfactors.org standard
for ATX.
"3.2.8. Capacitive Load
The power supply should be able to power up and operate normally
with the following capacitances simultaneously present on the DC
outputs. This capacitive loading should be used to check stability
and should not be included for noise testing.
Table 12. Output Capacitive Loads
Output Capacitive load (µF)
+12 V1DC 5,000
+12 V2DC 3,000
+5 VDC 6,000
+3.3 VDC 6,000
-12 VDC 350
+5 VSB 350
I suspect some motherboards probably use more than the limit, for
things like the +5VSB. But the above numbers would be used for
guidance, so someone doesn't slap 100,000 uF on one of the outputs.
So, the problem could be
1) Bad power supply - +5VSB is weak and dropping out, causing
PS_ON# to indicate shutdown.
2) Excessive total +5VSB loading from a DC load perspective.
3) A transient problem caused by a particular USB device. A
different device might not show the same characteristics.
A bad motherboard design, on the other hand, might do it
consistently for just about anything plugged in.
4) A miswired USB port, something getting shorted to ground.
Typically, miswired ports are on the front of the computer,
while the rear ports will always be correct (as they're routed
via copper tracks in the motherboard PCB).
HTH,
Paul
Nice explanation of the +5VSB, but it doesn't address
the OP's symptoms. Here's my interpretation of what
he said.
- Plug any USB device, including self powered devices,
(printer), into any USB port and the PC shuts down immediately.
- Plug a USB device into a USB extension cable connected
to any USB port on the PC and it works normally.
- Plug a USB device into a PCI USB card and the PC
shuts down immediately.
I was thinking of a software/driver issue, but the fact that it
works with a USB extension cable negates that. One other thing
could be static, and the length of the extension cable dissipates
the discharge. The problem with that is that is unlikey a static
discharge would occur every time a USB device is inserted
into the PC. The fact that the instant shutdown also occurs
when a device is plugged into the PCI USB card eliminates
the cause being in the motherboard USB controller.
.
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