IMX6D debug UART weird behaviour

[peterhygren]

Hi all,

Can anyone explain this behaviour:

When trying to boot various uboot images I can read out the debug info just fine. However, if I ever wanna input data to the UART, I get this weird voltage level shift that results in me having to reboot the board. This first image shows the TXD from the CPU (1.8V logic level) in blue. This gets interpreted by my USB-UART just fine as a normal boot log (that crashes, but that's another issue).

Now if I try to send data in the other direction (yellow) the data is read correctly by the CPU, but the output signal becomes distorted after the first pulse, and can no longer be read by my USB-UART because of the high LOW-level. See the image below:

All sub-sequent responses from the CPU is at this weird high level and I have to cycle power to remove the behaviour.

Has anyone got a clue what might cause this?

[peterhygren]

I've concluded that the reason for the behaviour is the dual drivers.

[fabio_estevam]

Can you post your the schematics that show the serial port connection (transceiver + serial connector) and also your code that does the serial pins configuration?

[peterhygren]

The data comes from the boot log on the debug UART. Sometimes, like 1 out of 20 or so, it doesn't hang like this, and I can actually talk to the uboot console.

The UART is 1.8V logic, and it's connected to our Wifi/Bluetooth module, and I've soldered two wires on the RXD and TXD.

RXD and TXD connections on the CPU.

Harness connection to Wifi module

Wifi module

And I also attached the CPU power schematic as a PDF.

[YixingKong]

Peter, if your question had been answered, please click Corret Answer/Helpful Answer.

Thanks,

Yixing

[LinWang]

Hi Hygren,

Do you mean you connect one UART port to WiFi and UART-USB debug at same time?

If yes, please disconnect WiFi connection and only use UART-USB debug. let's see if same issue happens again.

Thanks!