Redundant RC transmitter & receiver

I added a transmitter module to my Radiomaster TX16S successfully using crossfire protocol as shown on the images. The module communicates to a receiver using the same PCB. The firmware is also in one codebase, this makes the development easy because the same code works for both the transmitter and the receiver. 

You can read more about this project on my blog: https://bodrico.com/radiomaster-tx16s-redundant-link/.

Of course this is the part where I need more work. It's far from finished. At the moment basic functionality works:

• Both SX1280 modules are used. In transmit mode I alternate them while in receive mode they are both listening.
• Acquisition works: the receiver jumps to a random channel and scans through the channels 3 times slower than the transmitter. Eventually, the transmitter catches up with the receiver. In tracking the receiver adjusts its heartbeat timer to the received packets, to make sure that the transmitter and receiver will not drift out of sync.
• From the transmitter point of view every 4th packet is a downlink or with other words the receiver sends a telemetry packet.
• Communication with both the BMP388 and BNO055 works. I added a little C++ wrapper around the library that Bosch provided

In my github repo you can find all the pieces, but the whole firmware is not put together. When I'm working on one part I usually comment out some code. Eventually they all will be glued together...

I'm currently building and testing the rev.4 design. In previous revisions I had some hardware errors like wrong footprint for BMP388 and mixed signal on CAN and serial ports. I hope this version does not have any major errors any more. As of now the following hardware elements are validated to work:

• All power rails
• CPU
• Both SX1280 modules
• Servo signals
• BNO055 is working on 0x28 I2C address
• BMP388 is working on 0x77 I2C address

What I still need to test:

• Battery receiver measurement
• CAN bus
• GPS port
• Extension port