Drawing 500mA From USB Port: Voltage Drop Problem?

Question

HI there,

Currently I am using a PIC18F4550 from Microchip with their V1.3 USB firmware. I need to draw at least 300mA from the USB port through it for other components.

I changed the maximum power requirements of the device under enumeration to be 500mA instead of 100mA. Now, when I hook up a 20 Ohm load (which should draw around 250mA at 5V), the current is measured to be 221mA but the voltage has dropped to 4.5V. (With no load, the measured voltage on the rails of my board is 4.75V)

Curious, I changed the enumeration description back to 100mA max. I plugged in the USB, hooked up the load, and it still drew 221mA.

So the question is, how do I actually draw current from the USB port without the voltage drop that I have been seeing? I tried using another computer also, to see if it was a problem with the hub; it wasn't. And also, how come I can draw more than 100mA when I specified the device to only draw less than 100mA?

Answer 1

The USB bus can supply only 100mA except under special circumstances, see below.

by "enumeration" I assume you mean that your device requested more power from the PC, as noted below.

What you specify is the available current, not the current actually drawn. It means that if you specify 500 mA, the PC or hub will make up to 500 ma available to your load. How much current the load draws is determined by that load. If you specify 100mA and your load tries to draw more than 100mA, the voltage will drop.

I'm not sure that all hubs follow this part of the specification, so try connecting directly to the PC. In fact I'm sure they don't. If each port on a 8 port hub requested 500ma, that would be 4 amps from the PC, and it can't deliver that much power, the cables wouldn't handle it. Although I wouldn't be surprised to find PCs that fail that part of the spec also.

wikipedia:

The USB specification provides a 5 V supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines.[13] Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 2 mA up to a maximum of 500 mA.

If a bus-powered hub is used, the devices downstream may only use a total of four units — 400 mA — of current. This limits compliant bus-powered hubs to 4 ports. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer's operator when a given segment requires more power than is available.

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