5V or 3.3V on Spektrum VCC pad

[RyanEwen]

Hi again,

I received a board based on your design and have a question about the voltage of the Spektrum output. My flight controller isn't responding to the Spektrum output, so I am trying to figure out the issue.

What does work so far:

• The flight controller seems to recognize the presence of the board (it beeps and complains if absent).
• The LED flashes on the board. Bursts of fast flashes if TX is not turned on. Steady slow flashes if TX is turned on.

What doesn't work so far:

• The flight controller doesn't seem to recognize that I'm sending channel data.

Note: This is a non-configurable flight controller from Blade 350 QX3. It is only meant to work with included Spektrum satellite RX.

From reading about Spektrum satellite RXs I think they only work on 3.3V, and my flight controller does output 3.3V. A possible problem I see is that my multimeter reads 5V between Spektrum VCC pad and GND on the PCB based upon your design. I expected it to read 3.3V when connected to the flight controller. I assume the 5V reading comes through the board from SBUS on the other side.. Is this a problem?

Also, does this mean PCB is sending 5V Spekrum serial signal when maybe it should only be 3.3V?

Things I tried:

• Disconnecting Spektrum VCC line to flight controller. I don't think it's needed if the PCB is already 5V powered by SBUS, right?
• Connecting Spektrum VCC and disconnecting SBUS VCC in order to power the PCB using 3.3V. The board doesn't power up that way.

Tonight if I am able to I will try to read Spektrum serial output from PCB to make sure it is outputting data.

Thanks for any help.

[sebseb7]

if your board is not 5V tolerant, you can use 2 diodes to reduce the voltage. (is it still working)

the mega88pa can work at 3.3v if configured correctly.

compare the output to the output of your original spektrum receiver, there are different versions (resolution/
channels)

[RyanEwen]

I will compare output soon. I realized after looking at the source code that maybe Spektrum output could be DSM2 22ms when I want DSMX 11ms. None of the sellers specify the settings used when programming the chip.