Any experiences / current limits
andy at openmoko.com
Sat Aug 30 07:26:10 CEST 2008
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Somebody in the thread at some point said:
| I'm working on documenting various aspects of our USB port.
| We believe that the FreeRunner USB port should be capable of delivering
| the standard USB high current of 500mA. We also believe that we respond
| properly to USB negotiation of this current, up from the minimum of 100mA.
Just to clarify this, we should deliver 500mA OK AFAIK, and there is a
logical restriction of 100mA until the thing plugged in to us is
enumerated by us, but it is not enforced by current limit on our side.
We only have actual 500mA limit at all times. This is how all hosts I
saw do it, they do not enforce 100mA limit but the "real" 500mA limit.
Separately, when Freerunner is being the device and we are getting power
from a Host, that early 100mA limit is our problem to live up to and we
try to do that. But for reasons out of our control, from powerup
Freerunner PMU is wrongly configured with 500mA limit for some tens of
ms until the CPU starts and we set it correctly to 100mA. This brief
breaking of the rules doesn't seem to make trouble with any hosts
AFAICT... likely because they use same 500mA limit only strategy we do
when we are the host.
| I suddenly remembered that someone posted about using a USB keyboard
| that didn't work due to high current. However, the FreeRunner should be
| able to provide exactly as much current as a desktop PC, which I assume
| works with this keyboard.
| If any of you have experienced difficulty with the FreeRunner powering
| USB devices, please speak up.
USB keyboard should not be getting anywhere near 500mA, more like 50 -
100mA with the LEDs on, unless it has coffee warmer built in. So I
would ask how the reporter got from "USB keyboard not working" to
"overcurrent" specifically. The keyboards are low-speed USB devices,
1.5Mbps instead of 12Mbps, maybe there's something on that side.
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