Send `TL`/`TU` packets only in response to a request data `RD` packet

[produceconsumerobot]

In order to optimize and assess the time synchronization between the display device and the EmotiBit, a structured time synchronization protocol is employed in FW 0.2.0+ where TL/TU packets are only sent in response to a request by an EmotiBit. Specifically, time sync is initiated by the EmotiBit sending a Request Data RD packet with TL and TU in the payload. The EmotiBit then enters a state where all data sending and writing operations are suspended for a short period while it waits for the computer/phone to reply with either a TL or TU (or both) followed by an AK message with the packet number and RD in the payload. The processing of packets by the computer/phone should be optimized/prioritized to respond to these RD packets as fast as possible, targeting less than 10 milliseconds for a median round trip time.

The sequence is as follows:

• EmotiBit sends RD packet. Payload = "TL, TU"
• EmotiBit enters an acute listening state waiting for AK
• Computer/Phone receives RD request for a TL/TU packet
• Computer/Phone replies with TL or TU or both in separate packets.
• Computer/Phone replies with AK. Payload = [packet number of RD message], RD"
• EmotiBit receives and logs received messages and resumes normal data/packet processing

This protocol enables the EmotiBit timestamp to be accurately assigned (with error bars) to a local/UTC timestamp at approximately 1/2 the round trip time between the sending of the RD packet and receipt of the TL/TU packet. Further assessment of the unidirectional packet transmission delay between the TL/TU packet and the AK packet allows even better pinpointing of the timestamp synchronization between the computer/phone and the EmotiBit.