Abstract: A method and system for controlling an accessory function by a cellular telephone are disclosed. In accordance with the method, the cellular telephone generates a message payload comprising a command to set a selected register on an accessory device to a selected value. The cellular telephone transmits the message payload to the accessory device. The selected value is stored in the selected register on the accessory device in response to the message payload. The accessory device periodically reads the contents of the selected register. The accessory device controls the accessory function in response to the contents read from the selected register. The simple message protocol uses memory and bandwidth resources efficiently, and allows additional accessory functions to be readily controlled without significant modifications to the protocol.