Hardware Reference
In-Depth Information
as
high-power
USB devices and can use up to the maximum of five unit loads (500
mA) per port. They are rarely (if ever) labeled as such, and they look visually identical
to low-power (single-unit load) devices.
On top of all that, the dedicated USB connectors on the Raspberry Pi provide only one
unit load (100 mA) per port instead of the five unit loads that a “normal” computer
would. This amount isn't nearly enough to power a bus-powered hub with anything
else connected to it, so that won't work for you at all. The free (or extremely cheap)
USB 2.0 hubs? They are
always
bus powered. Sorry. You're going to have to buy
something a little nicer.
This is why if you connect a high-power USB device directly to the Raspberry Pi, it will
either attempt to operate in low-power mode (sometimes these devices can do that),
or the Raspberry Pi will simply power off or refuse to see the device. The majority of
high-power devices will detect at low power, then try to pull additional power when
put into active use (this is particularly common with wireless devices), resulting in a
confusing scenario where the device appears to work, and the Linux kernel drivers
load, but it doesn't actually work reliably or properly.
The solution to this problem space for the Raspberry Pi is to use an externally powered
USB hub. You will want to use a good one, though, because there are plenty of awful
choices here as well. It is common for the manufacturers of these USB hubs to cut
corners and design the hub to run off of a low-amperage power supply. They do this
because they assume that most of the devices you will connect to it are low powered
and that you will not have all of the ports used at once.
It is not uncommon for inexpensive, seven-port hubs to use a 1 A power supply. If each
of those seven ports is connected to a high-power (five unit loads, 500 mA) device,
they would need a 3.5 A power supply. More, really, because the hub needs power too!
To be safe, you should assume the opposite from what these cost-cutting manufac-
turers do. Just assume that any USB device you want to connect to your Raspberry
Pi is high powered and that each port in your USB hub will have a high-powered device
connected to it. Then it is a simple math problem to confirm if a USB hub will be a
good choice:
1. Take the number of ports on the USB hub, and add 1 (to account for the USB hub
itself).
2. Multiply that number by the size of a high-power load (.5).
The result will be the number of amps that the power supply for your USB hub should
be providing (at a minimum).
