An implantable medical device comprising a far field RF transmitter and receiver, a controller circuit communicatively coupled to the RF transmitter and receiver, and a wakeup timer circuit integral to, or communicatively coupled to, the controller. The controller is configured to initiate power up of the RF transmitter and receiver during a wakeup interval defined by the wakeup timer circuit, detect a digital key received from a second device during the wakeup interval, transmit a response using the RF transmitter when the digital key is received, and receive a communication from the second device and resynchronize the wake-up timer according to a time of the communication.