Gareth's Downstairs Computer
2018-03-04 12:45:22 UTC
you realize that the USB charger spec is 500ma, right?Wrong.
The below applies to A/B/microB sockets not USB-C sockets.
The original spec for USB said that what is now known as a Standard
Downstream Port can supply up to 500mA. This is a USB port that has full
data transfer capabilities as well as providing power. The standard USB
socket you see on a laptop etc. Normally the power is managed by the hub
controller with up to 500mA per socket.
Charging Downstream Ports can provide up to 2A as well as full data
facilities. You typically find one CDP amongst the sockets on most
Dedicated Charging Port is a charger with no data. This typically a wall
wart type of device. There are chargers that do not follow the DCP
standard but still work fine.
The USB charging spec provides a method for a device to talk to the port
to determine the charging characteristics before USB enumeration occurs.
I have a variety of wall wart chargers at 500mA, 700ma, 1A, 1.3A, 1.5A
and 2A. A smart device that implements the USB charging spec will pull
as much current as it needs up to the limit of the port type it
identified. So up to 2A for a proper CDP or DCP and 500mA for an SDP or