chris_c wrote:erm no I'm using a usb keyboard
Okay, I was confusing you with another user that said he was using a PS/2 keyboard.
chris_c wrote:are you sure the poly fuses are still involved when "injecting" 5v via pin 2 of the gpio (p1) connector?
are they not the other side of the micro usb regulation circuit?
Yes they are still involved.
5Volt distribution chain is:
PSU=>micro USB cable => micro USB port => 1.1 A main polyfuse (F3) => main 5V on board, which is to be found between testpins TP1 and TP2 and also beween both pin 2 and 4 (5V) and pins 6, 9, 14, 20 and 25 (GND) of the GPIO header.
From there it goes to the the two 140mA USB polyfuses F1 and F2, and from there to the two USB ports. If you do not draw any current from these ports you will still find the same voltage there as on TP1-TP2, but if you draw any current, the voltage will drop, in many cases with 5.0 Volt input you will find something like 4.6 to 4.4 Volt after the polyfuses, which won't do for USB devices requiring 4.75V, but is enough for some who work with 4.4 Volt.
Note that nowhere it this voltage distribution chain there is any regulation happening but from the main 5V voltage a number of regulators are fed, mainly the 3V3 regulator (RG2) and from that 2V5 and 1V8 regulators (RG1 and RG3), but also the SoC is itself a (switching) regulator that dynamically regulates its core voltage (that is the reason you can "overvolt" the SoC, in software, for over-clocking purposes).
but even the humble (3.3V fed) Ethernet&USB chip (IC2 the LAN9512 chip) has its own built in 1V8 regulator. so there is really a whole bunch of voltage regulation going on, just not for the USB ports.