Tony Firshman wrote, on 26/Aug/12 12:12 | Aug26:
Norman Dunbar wrote, on 23/Aug/12 18:37 | Aug23:
On 23/08/12 16:26, Tony Firshman wrote:
I find mine *does* need an external power supply when running some
keyboards.
Yes, the two USB ports are poly-fused to 100mA each. Some keyboards
and/or mice draw more so the fuses kick in and increase resistance (why
am I telling you this?) reducing current. This causes the keyboards to
act like a QL with a dicky membrane, and misses keys or start
auto-repeating at a huge rate.
I have a USB wireless keyboard and mouse for mine, a Perixx Periduo 707
Plus, and it too draws too much from the USB port.
Plugged into the Benq monitor's powered hub, though, it works as good as
it does on my work laptop - both show the same problems such as it hangs
unless I leave it connected after power up, for about 30 seconds - or I
can unplug and replug the wireless dongle to get it working fine again.
The Apple keyboard for instance wouldn't work at all when Pi
was run off my monitor usb. I know they say 700ma, but it can be a
*lot* less than that.
700 mA is the minimum required for the Pi, but the USBs are limited to
100 mA each.
... and the Pi to 750 ma.
The codes on the poly fuses are:
PSU - T075
USB - T014
ie 750ma and 140ma respectively.
... and I now see they are removed!
https://www.element14.com/community/message/59032
I will jumper them on my board, and I expect my Apple keyboard will
spring to life. Sounds a mite dodgy though, as people may add 500ma
devices. The Pi does not have any other protection.
Tony
--
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Voice: +44(0)1442-828254 Fax: +44(0)1442-828255 Skype: tonyfirshman
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