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USB Power

A standard USB Port (SDP: Standard Downstream Port) supplies power at "5 volts" and 100mA for standard (1.0 and 2.0) and 150mA for high speed (3.0) devices, called "Portable Devices" or PD's. E.g. if the port is USB3, you can pull up to 150mA. This power is available immediatly upon "connection" which is detected by the device pulling either D+ or D- pins to +5 via a 1.5 k ohm resistor. The actual voltage may be as much as 5.25v and as little as 4.40v from USB 2.0 or 4.45v from USB 3.0.

Additional power can be negotiated.

Suspension: That all only lasts 3ms. After that, the device is suspended and must reduce power consumption to insainly low levels on a standard port (exception is DCP, below). To avoid suspension, the device must send "keep alive" packets (see USB )

The use of a "Y" cable, with the exception of USB OTG Accessory charging adaptors (ACA), is prohibited. ^

Charging Ports

Another exception is a "Charging Port" as defined in the Battery Charging Specification 2010 ^ They supply between 0.5A at "5 volts" and (initially) 1.5 Amps before the voltage drops to 2.0 volts. In 2010, the spec was changed to support up to 5 Amps. There are several types:

DCP: Dedicated Charging Port:

Does not support data data. E.g. a USB power adapter. This port must short D+ to D-.

CDP: Charging Downstream Port:

Supports data transfer, and (initially) signaled this ability via a resistance under 200 ohms between the D+ and D- pins. High speed signalling was only supported if the current draw is reduced. In 2010, the spec changed and high speed data at up to 1.5 Amps was supported without any requirement for a signaling resistor.

Accessory charging adaptors (ACA)

Portable devices having an USB On-The-Go port may want to charge and access USB peripheral at the same time, but having only a single port (both due to On-The-Go and space requirement) prevents this. Accessory charging adapters (ACA) are devices that provide portable charging power to an On-The-Go connection between host and peripheral.

ACAs have three ports:

  1. the OTG port for the portable device, which is required to have a Micro-A plug on a captive cable;
  2. the accessory port, which is required to have a Micro-AB or type-A receptacle; and
  3. the charging port, which is required to have a Micro-B receptacle, or type-A plug or charger on a captive cable.

The ID pin of the OTG port is not connected within the plug as usual, but to the ACA itself, where signals outside the OTG floating and ground states are used for ACA detection and state signaling. The charging port does not pass data, but does use the D± signals for charging port detection. The accessory port acts as any other port. When appropriately signaled by the ACA, the portable device can charge from the bus power as if there were a charging port present; any OTG signals over bus power are instead passed to the portable device via the ID signal. Bus power is also provided to the accessory port from the charging port transparently.

ACA is identified in two ways:

  1. during USB idle state, D- is set to between 0..5 and 0.7 volts with up to 250uA draw (VDM_SRC)
  2. the ID pin is pulled to ground by a 122 to 126 k ohm resistor

RID_A 122 - 126 Kohm
RID_B 67 - 69 Kohm
RID_C 36 - 37 Kohm
RID_FLOAT > 220K
RID_GND < 1K

Very few devices are known to actually support OTG and charging at the same time, even when used with the proper ACA cable.

See also:


file: /Techref/io/usb/power.htm, 5KB, , updated: 2019/8/29 14:10, local time: 2019/12/12 07:46,
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