YD717 protocol    2013 Dec 30

Other names: Sky-botz UFO Mini

These quads use the Beken BK2421 Transceiver chip.  It is compatible with
the nRF24L01, with the addition of a special register set.

Data is transmitted every 8 msec using 8-byte packets.

Only a single channel is used - no frequency hopping.
[TODO: Stock TX always uses channel 3C.  Is this fixed? If so, why the
need for 60 bind packets.]

The tx/rx address is 32 bits.  [TODO: Stock tx always uses same value.
Is this fixed?]

# Binding #
After initialization, the transmitter sends 60 8-byte bind packets.  These
packets look like normal data packets with the trim values fixed at 0x32.  The
packets are unacknowledged so are retried 10 times each.  The tx address
is set to the same value that will be used later for data packets.

After the bind packets, the tx/rx value is changed to a common bind address.
A single packet is sent that includes the tx/rx address to use for further
communications.  The receiver acks this packet.

The tx/rx address is then set back to the chosen value and data packet
transmission begins.

# Data Packets #
Each channel is 8-bits. When high rates are selected the range is 00-FF.
When low rates are chosen the roll and pitch values are limited to 41-BF.

Trim values are sent in three separate bytes.  Trim ranges from 00-7F.

The flag byte is 00 in low rates and 0F in high rates.
[TODO: Is this an auto-flip control?]

## Example data packet
aa bb cc dd ee ff gg hh
00 80 3C 80 80 47 6C 00

0  aa: throttle (0 is full down)
1  bb: rudder (0 is full left)
2  cc: elevator trim
3  dd: elevator (0 is full down)
4  ee: aileron (0 is full left)
5  ff: aileron trim
6  gg: rudder trim
7  hh: flag