Hardware Reference
In-Depth Information
As mentioned previously in this hack, the Model B draws a peak current of 700 mA,
so if your USB cable has 5 ohms of resistance, it would result in a 3.5 V voltage drop.
As far as the Pi is concerned, that's huge.
The good news is that most USB cables don't have 5 ohms of resistance, and the really
good ones will be very close to 0. We haven't really had problems with cables that
came with modern phones, which seems to be most people's source of such cables.
If you need to purchase a USB Micro B cable, Mediabridge's “USB charging cables”
test with a low resistance and are available on Amazon. Adafruit's USB Micro B cables
also work fine in peak-current draw on the Model B.
Why Do USB Devices Cause My Pi to Reboot?
Hotplugging (plugging in a USB device when the Pi is already running) will
often cause the Pi to reboot. Plugging in the device causes a power spike,
which means a drop in power to the Pi, which leads to the reboot.
The Rev 1 board had two 140 mA polyfuses on the USB ports that prevented
this but caused other problems, so Rev 2 boards do not. If you do anticipate
the need to hotplug a device, do it through a powered USB hub.
Test Your Cable's Resistance
If you have a cheap power supply, you can almost guarantee that's your problem. But
if you think you have a problem with your cable and want to test its resistance, you
can either take the cable apart or you can use an accessible Micro USB Type B device
(something that has ground pins on it).
The USB Type A connector is big enough that you can get to the ground pin directly.
(It's Pin 4, the first pin on the left if you're looking down the cable with the hollow space
at the top of the connector.) Measure resistance with a calibrated multimeter set to
the lowest ohm setting from ground on the Micro USB-B connected device to ground
on the USB Type A connector to get a good idea of the cable's resistance.
You can also get a little hardware tool to simplify this. Bitwizard B.V. makes a USB
prodder just for this purpose.
Some people have even made their own cables by soldering low-resistance wires to a
power supply and a Micro USB Type B connector. This is a neat hack if you just want
to try making cables, but it's not really necessary.
 
Search WWH ::




Custom Search