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Power Usage Notes<br />
Warning: Adding peripherals may increase the loading on the power supply to your<br />
board and this, in turn, may affect the voltage presented to the <strong>RPi</strong>. If the<br />
<strong>RPi</strong>'s supply voltage falls below a certain value (anecdotally stated as<br />
around 4.75V), or it begins to fluctuate, your setup may become unstable.<br />
There is a Wiki section about this issue (http://elinux.<strong>org</strong>/<br />
<strong>RPi</strong>_Hardware#Power) which is worth a read.<br />
Model B Hardware Revisions and USB Power limits Hardware Revision 1.0 The<br />
original Model B board had current limiting polyfuses which limited the power output of<br />
each USB port to approximatly 100mA. USB devices using more than 100mA had to be<br />
connected via a powered hub. The PI's PSU was chosen with a power budget of 700mA<br />
of which 200mA were assigned to the USB ports, so the Raspberry Pi's (poly)fuses were<br />
designed only for devices up to 100mA, and typical 140mA polyfuses will have as much<br />
as 0.6 volt across them when drawing currents near the 100mA limit. As a consequence<br />
the USB ports are only directly suitable for "single current unit" USB devices which,<br />
according to USB specifications, are designed to work with just 4.4 Volt. Not only do non<br />
single current unit devices draw more current (causing greater Voltage drops, and greater<br />
stress on the fuses), they also might require 4.75 Volt to work.<br />
Model B Hardware Revision 2.0 and Revision 1.0 with ECN0001 change This had the<br />
polyfuses removed, removing the 100mA current limitation for each USB port (but<br />
leaving the main fuse F3 intact). Users should still ensure their power supply can power<br />
the <strong>RPi</strong> and the USB peripherals. Revision 2.0 was released in August 2012.<br />
Linux Driver Issues<br />
Shortly after the Raspberry Pi was released it was confirmed that there were a number of<br />
issues with the Linux USB driver for the SMSC95xx chip. These included problems with<br />
USB 1.x peripherals that use split transactions, a fixed number of channels (causing<br />
problems with Kinect) and the way the ARM processor handles the SMSC95xx<br />
interrupts. [1] (http://www.raspberrypi.<strong>org</strong>/phpBB3/<br />
viewtopic.php?f=28&t=12097&start=76) [2] (http://www.raspberrypi.<strong>org</strong>/phpBB3/<br />
viewtopic.php?f=28&t=5249&start=44) A large number of fixes were included in the<br />
2012-08-19-wheezy-raspbian Linux image.
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