Hummingboard Gate Mikrobus (UART3) RS485

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This post is, more or less, a continuation of something I started here, but adapted to Armbian. I will start with an overview of what I have done (maybe this is relevant) and finish with my questions.


What I've done:

I have a Hummingboard Gate with a Mikrobus RS485 click 3.3V (this one: In order to make this work I need to change a few things in de device tree.

The dtb that my board uses is 'imx6dl-hummingboard2.dtb' so I copy that file to my home directory for safe keeping.

cd ~
sudo cp /boot/dtb-4.19.18-cubox/imx6dl-hummingboard2.dtb .

Then I convert this dtb to a dts:

sudo dtc -I dtb -O dts -o imx6dl-hummingboard2.dts imx6dl-hummingboard2.dtb

I looked for the pwm1grp to find the phandle of the group that is using the pin I need to use as the RTS-pin. Using this phandle I find 'pwm@2080000' in which I change the status to 'disabled'. If I understand correctly, this will free the pin for RTS use (but I might be wrong... Device trees might as well be magic to me).


Next I find uart3. This is the uart that is on the Mikrobus-connector. For the clickboard I use, I need to swap RX en TX (don't ask me why). I change the uart3-entry from:

hummingboard2-uart3 {
	fsl,pins = <0x168 0x538 0x0 0x2 0x0 0x1b0b1 0x164 0x534 0x90c 0x2 0x0 0x40013000>;
	phandle = <0x4b>;


hummingboard2-uart3 {
	fsl,pins = <0x168 0x538 0x90c 0x2 0x1 0x1b0b1 0x164 0x534 0x0 0x2 0x0 0x1b0b1>;
	phandle = <0x4b>;

This switches the RX en TX function on the pins. Note that I do not completely understand the significance of the "0x40013000" and "0x1b0b1". Perhaps I have chosen the wrong value (see question 2 below).

Using the phandle in uart3, I find the serial-definition that I am using: "serial@21ec000". This is the serial to which I add definition for the rs485. I end up with:

serial@21ec000 {
	compatible = "fsl,imx6q-uart", "fsl,imx21-uart";
	reg = <0x21ec000 0x4000>;
	interrupts = <0x0 0x1c 0x4>;
	clocks = <0x4 0xa0 0x4 0xa1>;
	clock-names = "ipg", "per";
	dmas = <0x13 0x1d 0x4 0x0 0x13 0x1e 0x4 0x0>;
	dma-names = "rx", "tx";
	status = "okay";
	pinctrl-names = "default";
	pinctrl-0 = <0x4b>;
	rs485-rts-delay = <0x0 0xc8>;
	rts-gpios = <0x2f 0x1d 0x0>;

I've added the last 3 entries. The rest is unchanged.

I now compile the changed dts with:

sudo dtc -I dts -O dtb -o imx6dl-hummingboard2-changed.dtb imx6dl-hummingboard2.dts

And copy the result to the dtb-directory (See question 1 below).

sudo cp imx6dl-hummingboard2-changed.dtb /boot/dtb-4.19.18-cubox/imx6dl-hummingboard2.dtb

Now I reboot my board and the result is a board with a working rs485 connection.


I use this board to communicate with a modbus-rs485-device. Every now and then I see the following error message in syslog: "localhost kernel: imx-uart 21ec000.serial: DMA transaction error." Luckily this doesn't completely disturb the board. It is able to continue (see question 3 below).


My questions:

Question 1: Right now I am overwriting the dtb in /boot/dtb-4.19.18-cubox/ with my custom dtb. Is there a better way to do this?

Question(s) 2: Am I using the right values in the RX/TX-swap? Any idea how I would find this out?

Question(s) 3: What does the "imx-uart 21ec000.serial: DMA transaction error." mean? Any ideas how to fix it? (I've tried searching the web, but to no avail as of yet... as I said before: "Device trees might as well be magic to me").

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