Methods, devices, and systems for communicating with an implantable medical device of a last far field communication session during a subsequent far field communication session while using a same session key

An external device that communicates with an implantable medical device, comprising:

• a proximity communication circuit;
• a far field communication circuit;
• a controller coupled to the proximity communication circuit and the far field communication circuit, the controller being configured to:
• exchange a key with the implantable medical device over a first proximity communication,
• begin a first far field communication session with the implantable medical device and exchange communications during the first far field communication session with the implantable medical device where the implantable medical device implements the key when exchanging the communications,
• terminate the first far field communication session,
• attempt to exchange a second key over a second proximity communication after terminating the first communication session,
• attempt to begin a second far field communication session where the controller expects communications of the second session, including a response to the attempt, to use the second key,
• if no response is received to the attempt to begin the second far field communication session that uses the second key, then attempt to begin the second far field communication session where the controller expects the communications of the second session, including the response to the attempt, to use the first key, and
• if a response that uses the first key is received to the attempt to begin the second far field communication where the controller expects the response to use the first key, then conduct the second far field communication session where communications of the second session use the first key.

A system, comprising:

• an implantable medical device;
• and
• an external device, the external device comprising a proximity communication circuit, a far field communication circuit, and a controller coupled to the proximity communication circuit and the far field communication circuit, the controller being configured to:
• exchange a key with the implantable medical device over a first proximity communication,
• begin a first far field communication session with the implantable medical device and exchange communications during the first far field communication session with the implantable medical device where the implantable medical device implements the key when exchanging the communications,
• terminate the first far field communication session,
• attempt to exchange a second key over a second proximity communication after terminating the first communication session,
• attempt to begin a second far field communication session where the external device expects communications of the second session, including a response to the attempt, to use the second key,
• if no response is received to the attempt to begin the second far field communication session that uses the second key, then attempt to begin the second far field communication session where the external device expects communications of the second session, including the response to the attempt, to use the first key, and
• if a response that uses the first key is received to the attempt to begin the second far field communication where the external device expects the response to use the first key, then conduct the second far field communication session where communications of the second session use the first key.