The RF link is slow, and unreliable, so given that I came up the following scheme for transmitting the readings back:
- Only transmit readings when they change
- Transmit each reading 3 times when it changes
- Add a checksum so the receiver can tell if the reading was transmitted OK
The Manchester library I’m using (originally from here) just transmits 16-bit integers, so I want to pack everything into 16 bits.
The reading you get out the ADC on the ATTiny is 10-bits, and for now I’m transmitting the whole of it. The next 2 bits identify the axis the reading is for, and the final 4 bits are a simple checksum (XOR of each of the groups of 4 bits together).
The receiver checks the checksum, discards the reading if it’s wrong, and otherwise updates the axis and sends the reading out to serial (for now) so I can see what it’s up to.
Rather than pasting the whole source in here, I’ve put it up on github: Transmitter, Receiver.
Finally, a screenshot of it in action:
