Under household conditions the port capability can be checked by applying a big 10 Ohm (or 5.5 Ohm if USB 3.x) resistor to a stripped-off cable. The tester either applies a load equal to 5 units and checks if the voltage drop doesn't exceed specifications (5% or 10% margin), or applies a step-wise increasing load and determines at which point the (optional) overcurrent circuit trips over. If a USB system (host or hub) is declared as normal host, the ports are tested to USB-IF test specifications using specialized USB port testers. This limit is very inconvenient to customers, and is rarely enforced. However, if a USB host is a skinny battery-powered device (such as MP3 player or smartphone), this can be declared by manufacturer as "low-power host", and the USB port can be limited by design to deliver 100/150 mA only. Unfortunately, it fails to do so, and therefore is not USB-compliant. Or more, which is specified in particular PSU.įor example, if a Raspberry Pi3 gadget gets its power from AC-DC adapter from a wall AC power, it must supply at least 500 mA per each (of 4) ports. For example, a common desktop PC in sleep mode derives the VBUS power from +5VSB rail of its PSU, which at least is capable to deliver 2 A of current. So it could be more, unless an OPTIONAL overcurrent functionality is supported in hardware. So, if you have a desktop PC or laptop connected to AC outlet, each USB port MUST supply 500 or 900 mA of current. Supply at least five unit loads to each port. "Systems that obtain operating power externally, either AC or DC, must USB standard defines two classes of USB ports, "high-power ports", and "low-power ports" For USB 2.0 one unit is 100 mA, and for USB 3.x one unit is 150 mA. To start, the power delivery is defined in "units of load". Power that must be delivered by a USB port is defined in Section 7.2.1 of USB 2.0 Specifications.
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