1. Home
  2. Raspberry Pi Group
  3. New Pi...
  4. Page 3

New Pi...

I don't know much about the rPi, but the USB spec says the current drawn from a port will remain under some limit, I think it is well below 100 mA when initially powered on. It then has to register with the host as a higher current device before it can draw the higher current. So the current switch in the rPi is likely related to that. It is not clear to me if the switch in the rPi is to limit the current it draws (a poor way to do it as this will cause it to crash) or if this is for the current going out the USB ports where it is part of the spec for a host.

Actually, thinking about the numbers, I think this is the current drawn as a slave since the total current by spec should be no more than 500 mA on any one USB port. So their 600 mA limit is to allow the 500 mA to be reached but not significantly exceeded I suppose. Then if you know you are running from a PSU that can provide more power you can supply more to your downstream USB ports. Just speculating really.

--

Rick
Reply to
rickman
Loading thread data ...

The rounded corners let the board fit in a plastic case that has rounded inside corners which means it can fit more snugly.

Here are some...

formatting link

formatting link

Sometimes less is more....

formatting link

formatting link

The list goes on....

--

Rick
Reply to
rickman

The Pi SoC only has one chip select line. That is what is stopping it using the available 1GB chips as they all need two.

Reply to
Dom

What happens in-theory, according to the spec. and what happens in practice (usually dictated by the bean counters) are two separate things...

So yes, devices are supposed to negotiate power but in practice, even if they do, then the host-side may not have the right hardware support to actually realise it.

The Pi - model B/1 has a 140mA polyfuse on each port. That more or less limits the current to each port to a little over 100mA.

The B/1.1 and the B/2 removed those polyfuses - so the available USB current was more - up to what is left-over from the rest of the board off the main incoming 700mA supply (protected by a 700mA polyfuse)

The B+ has a 2 amp polyfuse on the incoming supply and a USB current limiter that allows a maximum of 600mA shared over all 4 ports, or, via a soft switch a maximum of 1.2 amps shared over all 4 ports.

I think (but I'd really have to look it up), that shorting the data pins together indicates a charging port and in that case a maximum of 2 amps is allowed to be drawn, however some peripherals take more than 600mA anyway - e.g. some USB hard drives when powering up.

I don't think there is any software support to automatically select the 1.2A limiter over the 600mA limiter, so right now it has to be done manually - at boot time, or via setting the appropriate GPIO line.

See here for some details:

formatting link

Gordon

Reply to
Gordon Henderson

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.