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USB Power requirements, a little insight
From all of the issues concerning the ability of some devices to work and others not to because of the power supply issue, I decided to take a look into the USB 2.0 Specification.
From section 7.2.1 Classes of Devices from the USB 2.0 Specification dated April 27th 2003:
The power source and sink requirements of different device classes can be simplified with the introduction of the concept of unit load. A unit load is defined as 100mA. The number of unit loads a device can draw is an absolute maximum, not an average over time. A device may be either low-power at one unit load or high-power consuming up to 5 unit loads (500mA). All devices default to low-power. The transition to high power is under software control. It is the responsibility of software to ensure adequate power is available before allowing devices to consume high-power.
ROOT PORT HUBS
Are directly attached to the USB Host Controller. Hub power is derived from the same source as the host controller. Systems that obtain operating power externally, either AC or DC, must supply at least five unit loads to each port. Such ports are called high-power ports. Battery-powered systems may supply either one or five unit loads. Ports that can only supply one unit load are termed low-power ports.
From this released specification it shows that the manufacture of a product that incorporates USB into their designs has a couple of different power options. They are not required to provide a HIGH-POWER port in battery-operated devices. This spec is available for down load at www.usb.org . Look for a heading of DOCUMENTS off to the left hand side. It is about 10Mbyte zip file.
From this my guess is that the design engineers at DELL designed the system to be a LOW-POWER port and thus are required to only provide 100mA per port. Sine the only way to get the USB LOGO on their systems is to adhere to these specs and be certified by the USB Certification program. So where is the problem.
Now for those of you who feel that you need to have the higher power have to consider where this power is going to come from. The battery in the laptop has a finite current capacity. Users have expressed concern that the battery only lasts a couple of hours as is. In my own limited running of my system, an I8500, a fully charged new battery gave me about 2 ½ to not quite 3 hours of operation. This was while watching a DVD.
Now for each high power device that equates to 2.5Watts of power. My 8500 comes with a 72Watt/hour battery. Since it runs about 2 ½ hours on a full charge that means it takes about 29 Watts per hour to run. Now this extra 2.5 Watts for your USB device is going to add about 9% to your power budget. This will reduce your run time to about 2 ¼ hours. So for each high-power device that you hang on the USB buss, you will decrease your run time approximately 9% per device. So if you have 2 devices that require this amount of power your run time has decreased to slightly less than 2 hours. See where this is heading?
This brings up another point of contention with some users. That is the one about the laptop getting quite hot while it runs. Speed is a function of current. You want more speed? Well the trade off is more current, less run time. By increasing the USB output to 500 mA will cause more heating in the system. As the power is drawn from the battery it traverse along traces in the motherboard. There is a function known as IR Drop. This means that a trace has a fixed non-zero resistance. As this extra current is drawn from the batter the traces that carry it get hotter due to the IR drop of the trace thus causing more heat that has to be dissipated by the system.
So bottom line is that DELL had to make some decisions. All of which are sound from an Engineering stand point. If you need to have these power hungry devices then get a powered HUB to put in series or go buy a DESK TOP system.
Hope this adds some clarification to the issue.
From section 7.2.1 Classes of Devices from the USB 2.0 Specification dated April 27th 2003:
The power source and sink requirements of different device classes can be simplified with the introduction of the concept of unit load. A unit load is defined as 100mA. The number of unit loads a device can draw is an absolute maximum, not an average over time. A device may be either low-power at one unit load or high-power consuming up to 5 unit loads (500mA). All devices default to low-power. The transition to high power is under software control. It is the responsibility of software to ensure adequate power is available before allowing devices to consume high-power.
ROOT PORT HUBS
Are directly attached to the USB Host Controller. Hub power is derived from the same source as the host controller. Systems that obtain operating power externally, either AC or DC, must supply at least five unit loads to each port. Such ports are called high-power ports. Battery-powered systems may supply either one or five unit loads. Ports that can only supply one unit load are termed low-power ports.
From this released specification it shows that the manufacture of a product that incorporates USB into their designs has a couple of different power options. They are not required to provide a HIGH-POWER port in battery-operated devices. This spec is available for down load at www.usb.org . Look for a heading of DOCUMENTS off to the left hand side. It is about 10Mbyte zip file.
From this my guess is that the design engineers at DELL designed the system to be a LOW-POWER port and thus are required to only provide 100mA per port. Sine the only way to get the USB LOGO on their systems is to adhere to these specs and be certified by the USB Certification program. So where is the problem.
Now for those of you who feel that you need to have the higher power have to consider where this power is going to come from. The battery in the laptop has a finite current capacity. Users have expressed concern that the battery only lasts a couple of hours as is. In my own limited running of my system, an I8500, a fully charged new battery gave me about 2 ½ to not quite 3 hours of operation. This was while watching a DVD.
Now for each high power device that equates to 2.5Watts of power. My 8500 comes with a 72Watt/hour battery. Since it runs about 2 ½ hours on a full charge that means it takes about 29 Watts per hour to run. Now this extra 2.5 Watts for your USB device is going to add about 9% to your power budget. This will reduce your run time to about 2 ¼ hours. So for each high-power device that you hang on the USB buss, you will decrease your run time approximately 9% per device. So if you have 2 devices that require this amount of power your run time has decreased to slightly less than 2 hours. See where this is heading?
This brings up another point of contention with some users. That is the one about the laptop getting quite hot while it runs. Speed is a function of current. You want more speed? Well the trade off is more current, less run time. By increasing the USB output to 500 mA will cause more heating in the system. As the power is drawn from the battery it traverse along traces in the motherboard. There is a function known as IR Drop. This means that a trace has a fixed non-zero resistance. As this extra current is drawn from the batter the traces that carry it get hotter due to the IR drop of the trace thus causing more heat that has to be dissipated by the system.
So bottom line is that DELL had to make some decisions. All of which are sound from an Engineering stand point. If you need to have these power hungry devices then get a powered HUB to put in series or go buy a DESK TOP system.
Hope this adds some clarification to the issue.
crippy59
7 Posts
0
October 3rd, 2003 06:00
Wow. Thanks for the detailed investigation.
However, I have to disagree slightly. A laptop is both battery operated and wall powered, so the regulation is not strickly defined.
IBM X31 notebook, for example, supplies over 600 mA to the usb.
Interestingly, Dell does have a high power version of usb port, it's called the Dbay.
crippy59
7 Posts
0
October 3rd, 2003 13:00
I just had an idea.
It's simple to build an adaptor - Dbay powered usb, using a $5 transformer chip.
If I sell it, I wander if Dell would sue me. :-)
Diagon Alleycat
23 Posts
0
October 3rd, 2003 22:00
dagrizz
34 Posts
0
October 3rd, 2003 23:00
Good reply. Maybe we might actually get to the bottom of it.
That is good to know. I wonder if it is the same for the I8500. This issue keeps coming up because other users have been complaining about not being able to connect a device that requires some where aproaching 500 mA and not getting them to work with their system until they connecting them through a powered hub.
You stated that you tested this with a feed through tap. Are you refernig to using a current meter or such in line with the USB devices that you were running. Do you know what the stated current draws are for the 2 fans and cooler?
Have a good weekend.
Mike
Networqs
78 Posts
0
November 22nd, 2003 14:00
Instead of just guessing, is there a way to find out what how much power the Inspiron USB port supplies?
I looked but couldn't find this information any where on Dell's website.
Will Sutton
41 Posts
0
November 23rd, 2003 01:00
chast
121 Posts
0
December 26th, 2003 13:00
The USB port information on my 5150 (in device manager) shows two USB ports with Power available to be 500mA per port.
That should be adequate don't you think?
dagrizz
34 Posts
0
December 26th, 2003 19:00
That sort of depends on what you are connecting to that port.
For a single device it might be. You should try it and see. If you are connecting more than a single device such as a hub, it might not be and you should probably use a powered hub. Here is why. The hub will require a portion of that 500mA for its operation internally. The remaining power will then be distributed to the devices connected to the ports on that hub.
So the short answer is probably. But you wont know until you try.
chast
121 Posts
0
December 26th, 2003 19:00
Hummmmm...see what you mean. My external mouse takes 100mA, (not all that much), but adding external keyboard and printer may start to push it. Hate to hook up another ac adapter for hub though. Oh well, so much for afterthoughts. right.
scorpionbyte
2 Posts
0
December 31st, 2003 00:00
Jinan
19 Posts
0
December 31st, 2003 02:00
You may not necessarily need an AC source to power your hub, or for devices that need to be powered. There are adapter cables out there that plug into the PS2 port which can power a hub or a USB device that needs power.
I have done this and it works fine. Only note of caution is to check the polarity of the jack that plugs into the device or the port as some manufacturers reverse the polarity so that you have to buy their power supply - Adaptec comes to mind! I have several USB devices that need external power and they all work and Adaptec is the only one with different polarity.
These adapter cables even have a feed through in the event you have a PS2 mouse or keyboard.
Pete
I8500 - P4-M 2.20G, 1GB RAM, UXGA
Win XP Pro, SP1, A05 BIOS
40GB HD, 40GB Media Bay HD, 40G USB 2.0 HD
Philips CD-R/RW/DVD+R/RW
D-Dock APR+D-Dock Stand
SyncMaster 152S
HP 990cxi, HP 7960, Epson 1650
dagrizz
34 Posts
0
December 31st, 2003 03:00
Jinan:
That might be a soultion, but you have to keep in mind that that power has to come from some where. In the case of a laptop that somewhere is the battery.
If you are running on battery alone, you have a finite source of power. By running power hungry devices and powering them off of a PS2 port you are just increasing the power draw from the battery and decreasing your available running time.
There is no free lunch.
Mike
Networqs
78 Posts
0
December 31st, 2003 10:00
Not all laptops have PS2 ports for additional power.
In any event, I consider that a makeshift solution only for emergencies.
USB devices should work better than that.
Jinan
19 Posts
0
December 31st, 2003 21:00
USB works fine. It is Dell that chose to not power their USB ports. The PS2 adapter is a work-around which will permit you to power the USB device without having to use or carry around an extra AC adapter. Yes it will increase drain on the battery, but I would rather carry around a very small and light cord instead of another AC adapter. In any case, even if the ports were powered, you'd still get the battery drain!
I8500 - P4-M 2.20G, 1GB RAM, UXGA
Win XP Pro, SP1, A05 BIOS
40GB HD, 40GB Media Bay HD
Philips CD-R/RW/DVD+R/RW
D-Dock APR+D-Dock Stand
SyncMaster 152S
HP 990cxi, Epson 1650
Will Sutton
41 Posts
0
February 3rd, 2004 02:00
Interesting thing I discovered. Have a card reader that wouldn't work with my notebook. It would partially see it but only assign it one drive letter . Anyway had it plugged into a hub ( non powered ) and it wouldn't work but when I rebooted the laptop with the hub already attached it worked. Tried unplugging the cardreader and reattaching and once again it wouldnt work correctly. Rebooted with card attached to USB and all the drives ( card reader has 4 types and each get a drive letter ) were there and it worked. Tried the same thing without the hub with the same results so the solution to getting my USB problem fixed is to boot with the device attached.
Also had a 128meg memory key that would only work if the USB mouse wasn't connected but if I boot with both in they then both work.
Very strange but I am sure someone here can explain