[ntp:questions] Do I have a lock to my NMEA GPS?

Ralph Aichinger ralph at pangea.at
Mon Nov 12 17:04:07 UTC 2012


Terje Mathisen <"terje.mathisen at tmsw.no"> wrote:
> I would be willing to (make a small) bet that the Pi has to support up 
> to the 500 mA of the 2.0 spec, simply because there are so many 
> peripherals, particuarly USB disks, that require this in order to work 
> at all.

No, no, the Raspberry Pi certainly does not supply 500mA. There is a
fuse in it that limits total current for the Pi itself and all connected
USB peripherials to 1.1A. The Pi itself needs a maximum of 700mA, so
if it runs at maximum power there are a maximum of 400mA for USB devices.
And that is the maximum, because the fuse has already a somewhat higher
resistance at 1.1A, so for weak power supplys the voltage will drop
further.

There are lots of devices that won't work in the Pi, e.g. most WLAN
adapters draw too much current.

> OTOH, it has to depend on the Pi power supply being big enough. Is there 
> a wall wart or similar in the kit?

You are supposed to buy a 700mA (minimum) 5V micro USB power supply.
In practice there are good 2A supplies available (for the iPad, e.g.)
but the Pi cannot use more than 1.1A from the USB port unless you 
remove or bypass the main fuse.

I really hope that in the next version they test and fuse it for
higher current (e.g. 2.1A, that is what iPad chargers supply).

Of course the obvious workaround is a powered hub, but that is
even more cable mess.

/ralph



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