A method carried out in a system having an active stylus and a sensor controller includes establishing frame synchronization between the controller and the stylus, selecting a first variable-length command from among multiple variable-length commands each including data of a variable number of bits, transmitting the first command in a first portion of a frame using an uplink signal having a variable time length that depends on a number of bits of the first command, receiving the uplink signal having the variable time length, detecting the first command by decoding the uplink signal up to a tail of the uplink signal, and transmitting a downlink signal that depends on the first command in a second portion of the first frame that is different from the first portion of the first frame.