Raspberry Pi Power Supply Micro USB B Specification

The Micro Universal Serial Bus (USB) type B, also known as Micro USB B, and Micro-B, is an interface connector for the purposes of connecting cellular mobile phones and portable devices to computers. It provides six electrical connections, consisting of five pins and a shell carrying the drain wire.

• Pin 1: +5 V
• Pin 2: Data-
• Pin 3: Data+
• Pin 4: ID
• Pin 5: Ground

Micro USB B Plug

The current two models of the Raspberry Pi require this type of plug for providing power. Only pin 1 and pin 5 are used for supplying power. Therefore, a proper power supply adapter must have a Micro USB B plug.

Descriptor File

USB is one of those wonderful inventions where you could plug devices to it, and it not only allows communication with the PC, but also provides power to the device without additional cables. All of this occurs seamlessly however the specification is rather involved. When you plug a device into the USB port of a PC, the operating system communicates with it to find out what it is and loads the necessary configuration settings for that device. A configuration file called "descriptor" is where the amount of power the device should receive is set. The power is set in 2 mA units up to a maximum of 500 mA.

If you were to connect the Pi to a computer’s USB port, the computer would not be able to detect it because the data pins are not used for communication with the operating system.

Low Power Bus Mode

Since the Raspberry Pi does not use the data pins, there is no enumeration, and no descriptor file setting the power requirement to high-power level. The device detection therefore always fails and the USB port defaults to low-power bus mode. In this mode, the device cannot draw more than 100 mA, and the voltage can go as low as 4.41 V.

Suspend Mode

There is also the added complication of "Suspend Mode" and "Suspend Current" functions, which is when a USB port enters suspended mode if there is no data activity on the port. During this time, a device connected to the USB port may not be able to draw any current from the port.

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