Problem with UART signal

Hi, I’m trying to connect an arduino to my APM module (ArduCopter) to get mavlink infos. The problem I have is about the UART signal.

If I plug my oscilloscope to the Tx of the APM, I have 0 to 5v signal normally. As soon as I plug arduino, the signal is 1.92 to 4.4v instead of 0 to 5.0. That is weird. I tried with a normal arduino and an arduino nano. The result is not the same but similar.

Any other device I plug, I have not this problem (uart to usb, telemetry, OSD, arm mcu).

Anyone have a clue ?

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Have you connected the ground of your AMP module to the ground of the arduino?

Yes, the ground is connected. I also tried to connect the 5v of the apm to power arduino instead of the usb. No différence.

In that case it looks like the APM module has a high output impedance so you should put a non inverting buffer on the input. Something like a 74HC07, or two inverting 74HC04s. Note you get 6 buffers in each package.

You are probably right. But, one of the module already connected to the APM is using an atmega328p and it's working well. So, I've done another test. I tried a bare atmega328p mcu and it works. So, I think there is something on the arduino board that cause the problem. Maybe it's the voltage regulator.

So, I'm wondering how people are developping code for modules of Arducopter...

Maybe it’s the voltage regulator.

No it is the fact that there is a USB to serial converter already connected to the serial pins through a 1K resistor. Anything driving these pins has to overcome this.

I see, that makes sense.
Thank for your answers.

fantasiiio:
Hi, I'm trying to connect an arduino to my APM module (ArduCopter) to get mavlink infos. The problem I have is about the UART signal.

If I plug my oscilloscope to the Tx of the APM, I have 0 to 5v signal normally. As soon as I plug arduino, the signal is 1.92 to 4.4v instead of 0 to 5.0. That is weird. I tried with a normal arduino and an arduino nano. The result is not the same but similar.

Any other device I plug, I have not this problem (uart to usb, telemetry, OSD, arm mcu).

Anyone have a clue ?

Simply use SoftwareSerial and 2 digital pins... Then use 1K series resistors inline with R & S. You could also use the 1K inline with the hardware UART just to see if this remedies your issue... You did not specify the specific board or clone.

Ray

It works with bare atmega328p mcu. I'm using usbasp to upload code and usb ftdi tu debug. At least I have something that works.

As soon as I plug arduino, the signal is 1.92 to 4.4v instead of 0 to 5.0.

If the device being connected to the arduino tx/gnd is not powered up, then the arduino may be back feeding the device and trying to power it.