POWERING USB DEVICES
This section covers both bus-powered devices and self-powered devices.
- A bus-powered device draws all of its power from the bus. It may draw up to 500 mA, if permitted to do so by the host.
- A self-powered device does not draw the bulk of its power from the bus, but rather from an external power supply.
Any USB device (or hub) can consume current from the bus only from its upstream port.
1-3-1 BUS-POWERED DEVICES
USB provides a power supply of between 4.75 and 5.25 volts and up to 500 milliAmperes to any device plugged into the port.
- Supply voltage can fall to a minimum of 4.35V at the device.
- When other devices are being plugged in, there can be transients lowering the supply voltage by 0.4V.
- Devices can be classified as bus-powered, self-powered, or hybrid-powered.
A USB device specifies its power consumption in units of 2 mA in the configuration descriptor. A device cannot increase its power consumption to a value that is greater than what it specifies during enumeration.
There are two classes of bus-powered functions:
- Low-power functions draw power from the VBus, and cannot draw any more than one unit load. The USB specification defines a unit load as 100mA. Low-power functions must also be designed to work down to a VBus voltage of 4.40V and up to a maximum voltage of 5.25V as measured at the upsteam plug of the device. For many 3.3V devices, LDO regulators are mandatory.
- High-power functions draw power from the VBus, and cannot draw more than one unit load until it has been configured, after which it can then draw up to five unit loads (500mA Max), provided this is specified in the descriptor. High-power functions must be capable of being detected and enumerated at a minimum 4.40V. When operating at a full unit load, the VBus provides a minimum of 4.75V and a maximum of 5.25V. Once again, these measurements are taken at the upstream plug.
A device which has been suspended, as a result of no bus activity, must reduce its current consumption to 0.5 mA or less. If a device is configured for high power (up to 500 mA), and has its remote wakeup feature enabled, it is allowed to draw up to 2.5 mA during suspend.
1-3-2 SELF-POWERED DEVICES
Self-powered functions are devices that require more than 500 mA of current. These devices may draw up to one unit load (100 mA) from the bus, and must derive the rest of its power from an external source. Should this external source fail, the device must have provisions in place to draw no more than one unit load from the bus.
The one unit load from the bus allows devices to be detected and enumerated without requiring external power.
Powered USB (also known as Retail USB, USB Plus Power, and USB + Power) is a proprietary variant of USB that adds new powering modes to the USB specification.
Powered USB uses standard USB signaling, but with the addition of extra power lines. it is commonly used for point-of-sale terminals. IBM owns the intellectual rights to Powered USB and charges a licensing fee for its use.
Powered USB can provide:
- +5 volts DC at up to 6 amps per connector (up to 30 watts)
- +12 volts DC at up to 6 amps per connector (up to 72 watts)
- +24 volts DC at up to 6 amps per connector (up to 144 watts)
1-4 BUS STATES
As there are two data lines, there are a variety of different conditions that can be signaled.
When no device is plugged in, the host will see both data lines low, as its 15 kohm resistors are pulling each data line low.
When the device is plugged in to the host, the host will see either D+ or D- go to a '1' level, and will know that a device has been plugged in. The '1' level will be on D- for a low speed device, and D+ for a full (or high) speed device.
The state of the data lines when the pulled up line is high, and the other line is low, is called the idle state. This is the state of the lines before and after a packet is sent.