I spent a couple of days looking at all sources that could be causing the problem and even flew it with it's own highly filtered 2.85 volt supply off its own battery and I was still having problems with it dropping out. I finally upped the voltage to 3 volts and everything was fine. Most of the M4 runs at 2.85 volts but we have a step/up for CAN bus and the second Spektrum receiver of 3v3 volts. The fix is to jumper that voltage to the receiver. The obvious question is why didn't we do this originally and it's simply because the 2.85 volts is better filtered and well within it's recommended voltage range. This seems to be a good fix but as always be cautious, even these little guys can do harm especially to an unprotected eye.
The fix is three parts, 2 hardware and 1 software all easy to do. At boom 2 their is a jumper J17 which supplies power to the onboard RX. If it has a solder jumper that needs to be removed to take the 2.85vdc supply away from the receiver chip. Then a jumper needs to be installed from the receiver side of J17 to the Spektrum Plus terminal. This provides 3v3 volts to the receiver chip instead of 2.85 vdc. I have included a couple of photo's showing you 2 options. One is from the pad I mentioned to Spektrum plus. A shorter connection would be from C33 to the Spektrum Plus terminal. Which ever is easier seems to work fine.
Caution, make sure you remove the J17 jumper as you would be putting 3v3 volts on the 2.85 volt buss if you leave it jumpered and it may do damage
The increase in voltage causes the baud rate from the receiver to go up to what I measured as 134078 baud. This is changed in spektrum.h from 115200 baud. A compiled hex and bin file will be provided but if you can handle the fine soldering and can recompile you'll be much happier with the receiver's performance.
The performance of an external Spektrum receiver of course is fine, we have 3v3vdc going to those pads which after the mod will power both Spektrum receivers.
After doing a little research it would be wise to use either the onboard RX or the external Spektrum but not both because we could reach the current limit of a regulator. If you are using a external Spektrum then you don't need to put the jumper
Cheers
Steve
- M4ReceiverJumper2.JPG (74.49 KiB) Viewed 15445 times
- M4receiverJumper.JPG (97.25 KiB) Viewed 15445 times