Methods, systems and apparatus for determining whether an accessory includes particular circuitry

A method of determining whether an accessory includes particular circuitry, comprising:

• measuring, at a host device coupled to an accessory, a first voltage received from the accessory, the first voltage being received via a power pin provided in the host device;
• sinking, at the host device, current from a power source via the accessory;
• measuring, at the host device, a second voltage received from the accessory, the second voltage being received via the power pin provided in the host device;
• and
• determining, by the host device, whether the accessory includes particular circuitry based on the relationship between the first voltage and the second voltage.

A host device comprising:

• a power pin operable to receive a voltage from an accessory;
• a data pin operable to communicate an instruction to the accessory;
• and
• control circuitry operable to:
• measure a first voltage received from the accessory via the power pin;
• send an instruction to the accessory via the data pin, the instruction instructing the accessory to alter an impedance, at the accessory, of a power path between a power source and the host device;
• sink current from the power source via the accessory;
• measure a second voltage received from the accessory via the power pin;
• and
• determine whether the accessory includes particular circuitry based on the relationship between the first voltage and the second voltage.