USB Power Limits

rustybr · 3rd March 2009

I have a quick question about usb power limits. I am currently running an Intel d845epi motherboard with an Intel 82801DB/DBM usb 2.0 enchanced host controller - 24CD. Three other Intel usb universal host controllers are listed. I have 4 usb ports on the back of my case + 2 font usb ports. My problem is that I have an external HD (with its own power supply) connected to a back usb port. The device is currently requiring 100mA. However, I also have an ipod which charges via the usb port it is connected to. When I connect the ipod the controller shows the external hd at 100mA and the ipod at 500mA. It doesn't matter which port I plug the ipod into, I will lose my read/write access to the external hd. I am assuming I am exceeding the 500mA limit, and windows is managing my usb ports by cutting the access to the external hd. But I was under the impression that the 500mA limit was per usb hub (and my device manager is showing 4 root hubs and 4 controllers). Would it be that all 6 of my usb ports are using the same power and thus limited to 500mA for all 6? I have also connected other devices (flash drives and usb mice) that require less current (80 - 100 mA) and my external hd works fine. I am currently at a loss, unless all 6 of the ports are limited to 500 mA collectively. Any thoughts?

**Bursley** · 3rd March 2009

It certainly sounds like they are sharing the same hub / power. Typically Windows USB drivers and / or additional drivers control the power to the USB device when it is connected to the hub. hard drives | USB disks for example shouldn't require an extra driver, but a device that has a charging method, such as an IPOD needs an additional driver to raise the voltage of the port, allowing the devices to charge properly. Its possible there is something wrong with this driver. Since I don't have an IPOD, I'm not sure if there is an additional driver, but I do know my Blackberry requires one to charge via the USB port.

rustybr · 3rd March 2009

There are no specific drivers for an ipod. And when I connect the ipod, it works perfectly; it just causes errors in my other usb mass storage devices. And what is really weird is that the device manager says that all the devices are functioning properly. However, I get a delayed write failure on my external hd. I understand why I am losing my external hd if I only have one hub (calling for more current than the 500mA limit). But it shows that I have multiple hubs. I have reinstalled the usb host controller and hub drivers multiple times, checked my bios for usb 2.0 and neither have helped. Also, these are all ports that are hard wired to the motherboard. I was just wondering if there is anything I am missing before I get a powered external usb hub or a pci - usb controller.

**TonyT** · 3rd March 2009

Are ALL of the MB hubs being used? Often a pc will have additional usb hubs on the mb that are not wired to anything yet.

**Arie** · 3rd March 2009

I had a look at the specs of your board & it says that there is a maximum of 500 mA current draw per USB port.

I would just get a powered hub, they usually cost around $15-$25.