If both transmitter and receiver frequently change channels on the same sequence,

[desertkun]

what stops them from reaching situation where they never actually catch each other and constantly jump on the same sequence, but with shifted index of the sequence array?

[philj404]

The interesting part is in the file RX_FHSS.c, in the first half of loop(). The boolean fhss_on keeps track of whether the receiver is synchronized with the transmitter yet.

• The receiver does not hop by default. It listens on a channel until the transmitter sends something on it.
• Sometimes a 2.4GHz channel might NEVER work -- for example when the receiver is right next to a busy WiFi base station. If the receiver never sees a valid packet in a certain amount of time (several cycles) it gives up on that channel and tries listening on the next channel.
• Once it sees a valid data packet, the receiver starts hopping at the same rate and sequence as the transmitter.
• If the receiver hasn't seen valid data "in a while" it starts searching again.