using a powered usb port to power a RPi

[vince]

Under the rash assumption that I will not be drummed out of the corps .....

Wound it be practical, feasible or safe to use a powered USB multi-port device to power a RPi and use that same device for the keyboard, mouse, etc.?

[bredman]

This seems to be a popular configuration, but make sure the power supply can give enough juice.

I would recommend a minimum of 1Amp (1000mA) in this case.

This may not work with some cheap USB hubs.

You can't do any real damage this way.

[vince]

bredman,

Thank you. I am waiting for my RPi. I got to the initial order w/ the rest of the masses, so I wait.

Vince

[MrEngman]

I'm currently using a Belkin 4 port F5U224 powered hub without any problems. It has been running pretty much 24hrs/day for about a week now.

The hub's host port is connected to the Pi's USB port. A cheap keyboard and mouse I think I got from Asda are plugged into the hub. A Micronet Wireless USB adapter (uses Realtek rtl8188cus) from Maplin is plugged into the second USB port on the Pi. A USB cable with microUSB connector, from Maplin, is plugged into the hub and the power connector on the Pi to supply the power to the Pi. This cable is really far too long, 1.5m, for a power cable and I would like a shorter one, much less than 0.5m if possible, but it hasn't caused any major problems even though it is so long.

I currently do not have an HDMI capable monitor/TV so I am using the RCA composite video to connect to my old PAL TV so I do not know if it will continue working ok once I have HDMI connected. Will almost certainly depend on the power drawn from the HDMI.

Software is debian6-19-04-2012. I have updated this using info at http://www.ctrl-alt-del.cc/201.....s-ada.html and it is proving to be very stable now. Before the software update I did see occasional issues with repeated characters using the keyboard and the wireless appeared to hang a couple of times although the Pi did not totally hang. All those issues have now disappeared and its running very well.

I am accessing the Pi at the moment using SSH across the wireless network using Putty on a Windows 7 laptop and I have several other PC's constantly pinging the Pi to keep the wireless active to see how stable it is. I have had NO problems of any sort since upgrading the software about 48hrs ago.

A note for all those people muttering about power issues, as my Pi is running very stable without any problems.

I have measured the voltage on the board, across TP1-TP2, and it is about 4.65v-4.68v, really much lower than I would like it to be. YES that is 4.65-4.68v. The voltage at the USB power connector is 4.80v so there is about 0.15v drop across the power fuse, something people might like to take note of. I have not been able to measure the voltage out of the hub yet so do not know the voltage drop across the 1.5m of cable but it's likely to be a fair amount. This is why I am particularly keen to shorten the power cable.

I have spent the last 30+yrs doing hardware design, the last 10 including a considerable number of embedded systems using various PowerPC chips, SCSI, ethernet and fibre channel so I think I know a little about power requirements;) and i'm finding the Pi is quite impressive in that respect.

However, for those with apparently unstable systems who suspect power issues I would suggest it would be a good idea to update the software as mentioned above.

All I need now is to get the HDMI connected - waiting on Asda to deliver my new HD TV - hopefully today or tomorrow. Will post an update to let you know how it behaves then.

Have fun, enjoy using your Pi.

[MrEngman]

A quick addition to my previous post.

The wireless hub I'm using is the N150 version Maplin order code N47HW as I noticed there are a couple of Micronet wireless USB adapters available.

[MotoBlue]

The shortest micro USB cable I have found is the 0.3m ASY-18685-001 Blackberry cable - plenty on Ebay.

[MrEngman]

I tried a cable from my Samsung USB powered DVD drive ~60cm long but very substantial. It also has a second USB connector for addition power but I didn't connect that. This reduced the voltage drop through the cable by 0.2 volts.

I have now got my HDMI TV and connected that. With my original configuration - see earlier post -
Pi still works reasonable well but I've seen a few funnies with the keyboard repeating characters - probablt voltage problems on the USB. Voltage across TP1-TP2 had reduced slightly and was about 4.6v. Bur even with so low a voltage Pi still behaved reasonable well.

I have, however, now change to using a small switched mode PSU capable of providing about 3A. It allows me to adjust the voltage and so TP1-TP2 is now ~5v and everything is running really well.

[jasonclark]

Is the potential (sorry... ) reduction on 5V supply a problem? I assume the core parts are on 3v3 or less?
Would the 5v rail only showing 4v6 cause issues with USB devices (wifi/flash memory)?

I guess that leads to a question - does the PI know the incoming 5V supply level?