An implantable medical device, external unit, and method for establishing communication between them are disclosed. The external unit learns the frequencies of transmission and reception for the implant. The transmit frequency of the implant is learned by the external unit measuring a difference between the transmit frequency of the implant and the external unit's receive frequency. The receive frequency of the implant is learned by the measured difference between its transmit frequency and the receive frequency of the external unit when the implant has a fixed transmit and receive frequency difference. Otherwise, the receive frequency is learned by the implant measuring the difference between its receive frequency and the transmit frequency of the external unit and by sending an indication of the difference to the external unit through the return signal. The external unit may anticipate a beginning of the implant's preset communication period based on its internal clock.