How to assign UART to different pins

[DiBosco]

If I understand correctly, you can pretty much assign any pin to any peripheral. I've been using a modded central uart project on a dev board but on this other board I'm using, I don't have access to the same pins. I think the pins are assigned in the dtsi file as follows:

&pinctrl {

uart0_default: uart0_default {

group1 {

psels = <NRF_PSEL(UART_TX, 0, 6)>,

<NRF_PSEL(UART_RTS, 0, 5)>;

};

group2 {

psels = <NRF_PSEL(UART_RX, 0, 8)>,

<NRF_PSEL(UART_CTS, 0, 7)>;

bias-pull-up;

};

};

Looks right as I am connected to P0.06 and P0.08 for the UART.

On the one hand you'd think, yeah, just mod this, but this is in

the ncs/zephyr directory so I'm assuming you don't want to mod this.

Again as far as I understand (but might not be right) you need to have some sort of other overlay file?

Is that right? If so (well, even if not), how exactly do you do this

and what exactly what I put in whatever place I need it? What if I needed to us, say P0.09

and P0.10?

Thanks.

[Edvin]

Hello,

You are correct. It is recommended to leave everything in the SDK untouched. I prefer to, whenever I want to modify anything in a sample, even, to copy it outside the SDK and then run it from there. Copy the sample folder to a custom location, then clidk "Open an existing application" in VS Code:

So to your actual question. You are on the right track. You should use an overlay file. There are a couple of different ways to do this. The most common way is to create a file with the board name that you are building for, such as:

nrf52840dk_nrf52840.overlay

if this is present in your application folder, or in a folder called "boards" that is present in your application folder (the same folder as the CMakeLists.txt), then the compiler should pick it up (you can verify it from the build log):

So in this file, you can specify everything you want to behave different from the default dts/dtsi files. So everything you want to change, you copy that block from the dts(i) file, paste it in your overlay file, and modify it there, such as:

&pinctrl {
	uart0_default: uart0_default {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 09)>,
				<NRF_PSEL(UART_RTS, 0, 11)>;
		};

		group2 {
			psels = <NRF_PSEL(UART_RX, 0, 10)>,
				<NRF_PSEL(UART_CTS, 0, 12)>;
			bias-pull-up;
		};
	};

	uart0_sleep: uart0_sleep {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 09)>,
				<NRF_PSEL(UART_RX, 0, 10)>,
				<NRF_PSEL(UART_RTS, 0, 11)>,
				<NRF_PSEL(UART_CTS, 0, 12)>;
			low-power-enable;
		};
	};

NB: You would want to change the uart0_sleep as well as uart0_default

NBNB: pin P0.09 and P0.10 are by default the NFC pins on the nRF52840. This needs to be disabled to make them work as normal GPIOs. For this, please see this ticket:
Can the NFC port be disabled and used as GPIO?

Best regards,

Edvin

[DiBosco]

Hi, finally back on this again. Thanks for the reply. 

An awful lot I'm still failing to grasp. 

So, in no particular order:

1. What is this uart sleep all about? 

2. That link about disabling P0.09 etc, I could not follow. There was something about meerkats, but what the actual solution is I do not get. 

3. Where is this arcane syntax explained? Let's take this for example:

uart0_default: uart0_default {
        group1 {
            psels = <NRF_PSEL(UART_TX, 0, 09)>,
                <NRF_PSEL(UART_RTS, 0, 11)>;
        };

a, Why is uart0_default repeated after a colon? 

b. what is the "group" thing all about? 

c. What does psels mean? 

Having trawled through loads of other people asking similar questions I am completely failing to compile with simple GPIO reassignments for LED driving. I can change the dts file and it works fine, but the overlay file, not a chance. I've tried a zillion different combinations. 

My app.overlay file which was already in the project and seems to be used according to the check above you mentioned:

/ {

chosen {

nordic,nus-uart = &uart0;

};

gpioCustom {

green_led_pin: green_led_pin {

gpios = <&gpio1 10 GPIO_ACTIVE_LOW>;

label = "green_led_pin";

};

};

aliases{

grn_led_pin = &green_led_pin;

};

};

In main.c

#define LED_GREEN_NODE DT_ALIAS(grn_led_pin)

static const struct gpio_dt_spec green_led = GPIO_DT_SPEC_GET(LED_GREEN_NODE, gpios);

I am down to one error after loads of combinations of things:

[{
    "resource": "/home/robertw/Software/Nordic/custom_service/central_uart/src/main.c",
    "owner": "C/C++: IntelliSense",
    "code": "20",
    "severity": 8,
    "message": "identifier \"__device_dts_ord_DT_N_ALIAS_grn_led_pin_P_gpios_IDX_0_PH_ORD\" is undefined",
    "source": "C/C++",
    "startLineNumber": 55,
    "startColumn": 46,
    "endLineNumber": 55,
    "endColumn": 62
}]

Or, more succinctly:

__device_dts_ord_DT_N_ALIAS_grn_led_pin_P_gpios_IDX_0_PH_ORD\" is undefined

What do I need to do here please?

It shouldn't be this difficult

to toggle a pin. :-(

[Edvin]

DiBosco said:

1. What is this uart sleep all about? 

If you are ever to disable the UART, then this says what state the used GPIOs should use in this state. Even if you never disable the UART (in the case where you always want to be able to receive UART messages, which are asynchronous, you can not disable it), then these still need to be defined in your devicetree (.dts/.dtsi/.overlay files).

DiBosco said:

2. That link about disabling P0.09 etc, I could not follow. There was something about meerkats, but what the actual solution is I do not get. 

Sorry if I was unclear. You said: "What if I needed to us, say P0.09 and P0.10?"

So I am not sure whether you want to use exactly P0.09 and P0.10, or if that was just an example, and it might as well be P0.11 and P0.12. 

If it was just an example, and you might as well use P0.11 and P0.12, then you can just copy the devicetree snippet describing the pin numbers from the .dts/.dtsi files and add it to your .overlay file, and change it there. 

If you want to use P0.09 or P0.10 specifically, you need to specify that these are not to be used as NFC, so you need to add CONFIG_NFCT_PINS_AS_GPIOS=y  to your prj.conf. Then set them in your .overlay file.

DiBosco said:

3. Where is this arcane syntax explained? Let's take this for example:

uart0_default: uart0_default {
        group1 {
            psels = <NRF_PSEL(UART_TX, 0, 09)>,
                <NRF_PSEL(UART_RTS, 0, 11)>;
        };

You can have a look here:

https://docs.nordicsemi.com/bundle/ncs-latest/page/zephyr/build/dts/intro-syntax-structure.html

But usually, you don't need to write these from scratch. Find what you want to change in the default board files, copy the parts that you want to change into your <board_name>.overlay file, and make the changes there. 

DiBosco said:

a, Why is uart0_default repeated after a colon? 

It is the syntax developed by Devicetree (see link above).

DiBosco said:

b. what is the "group" thing all about? 

It is just how it is defined in some yml file in Zephyr. Typically one group for input pins and one group for output pins.

DiBosco said:

c. What does psels mean? 

Pin selects. Just a syntax to map the correct pin accross all ports (P0, P1). Espesially useful for chips with many ports (P0 .. P9).

DiBosco said:

What do I need to do here please?

I suggest you make it a bit easier for yourself, and don't try to create your own custom "gpioCustom", but use the defined structure "leds" for output GPIOs (and "buttons" for input GPIOs). 

Something like:

// in <board_name>.overlay:
 / {
    leds {
		compatible = "gpio-leds";

        green_led_pin: green_led_pin {
            gpios = <&gpio1 10 GPIO_ACTIVE_LOW>;
            label = "green_led_pin";
        };
		
		led0: led_0 {
			gpios = <&gpio0 3 GPIO_ACTIVE_HIGH>;
			label = "sample_pin";
		};
    aliases {
        grn_led_pin = green_led_pin
		led0 = &led0;
	};
};


//in main.c:
#define LED_GREEN_NODE DT_ALIAS(grn_led_pin)
static const struct gpio_dt_spec green_led = GPIO_DT_SPEC_GET(LED_GREEN_NODE, gpios);

static void configure_gpio(void)
{
    int err = 0;
    // Output pins:
    
    err = gpio_pin_configure_dt(&green_led, GPIO_OUTPUT);
    if (err) {
        LOG_ERR("couldn't configure, err %d", err);
    }
    
    gpio_pin_set_dt(&led0, 1); // turn on green LED.
    
}

I believe something like that should work. If it doesn't, please upload what you currently have, and I can have a look at something that I can build and test.

Best regards,

Edvin

[DiBosco]

Hi and thanks for that. After eventually fixing a few overlay file issues from your suggestion I am still getting exactly the same problems. 

2313.central_uart.zip

Hopefully this zip will help. Line 55 in main.c has that same 

__device_dts_ord_DT_N_ALIAS_grn_led_pin_P_gpios_IDX_0_PH_ORD\" is undefined

Issue

Thanks!

[Edvin]

What NCS version are you using? I tried building it in v3.1.0, and it gave me this error, not the one that you saw:

When I changed the app.overlay:

/*
 * Copyright (c) 2023 Nordic Semiconductor ASA
 *
 * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
 */

/ {
	 chosen 
	 {
		 nordic,nus-uart = &uart0;
	 };
	 leds 
	 {
		compatible = "gpio-leds";

        green_led_pin: green_led_pin 
		{
            gpios = <&gpio1 10 GPIO_ACTIVE_LOW>;
            label = "green_led_pin";
        };
		

	};

	aliases 
	{
		grn-led-pin = &green_led_pin;
		led0 = &led0;
	};

};

only on line 27, changing:

grn_led_pin = &green_led_pin;

to

grn-led-pin = &green_led_pin;

And then it built fine. (some warnings regarding arguement types).

If you don't see what I am seeing, please let me know what NCS version you are using, and what your build command/build configuration is.

Best regards,

Edvin

[DiBosco]

Hi,

OK, that does work, thank, you, but what on earth is that all about? I have used underscore in C for decades and never come across an issue with it before. Why can you not use underscores here? 

The other thing that makes no sense here is that grn-led-pin  isn't actually used anywhere, so that is that all about? What is the point of the alias? 

[Edvin]

I agree that this is a bit confusing. The DeviceTree language is different from C. You can read about the syntax documentation here, and it is also discussed on DevAcademy.

And yes, it is confusing that you can't use underscore there, but when you translate it into C, to reference it, all "-" are translated to "_". 

One neat trick is, if you are using the nRF Connect for VS Code extension, you can right click elements in the devicetree files (.dts/.dtsi/.overlay), and select "Copy C Identifier". 

It is a bit sensitive to what you right click, but e.g. if I copy the grn-led-pin:

This is what is copied into the clipboard:

DT_PROP(DT_PATH(aliases), grn_led_pin)

which you can use in your C-files.

Best regards,

Edvin

[DiBosco]

OK, next question, what about input pins. I've spent another day trying to get this working. 

I've added this to the overlay file.

buttons

{

compatible = "gpio-keys";

sel_sw_pin: sel_sw_pin

{

gpios = <&gpio0 29 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;

label = "sel_sw_pin";

};

};

This is now the alias section

aliases

{

grn-led-pin = &green_led_pin;

led0 = &led0;

blue-led-pin = &blue_led_pin;

led1 = &led1;

red-led-pin = &red_led_pin;

led2 = &led2;

sel-sw-pin = &sel_sw_pin;

};

But I get an error on the second line below:

#define SWITCH_NODE DT_ALIAS(sel_sw_pin)

static const struct gpio_dt_spec user_switch = GPIO_DT_SPEC_GET(SWITCH_NODE, gpios);

With this familiar friend:

message": "identifier \"__device_dts_ord_DT_N_ALIAS_sel_sw_pin_P_gpios_IDX_0_PH_ORD\" is undefined",

It clearly is defined. :-(


Also, I've tried adding this to the overlay:

&uicr

{

nfct-pins-as-gpios;

};

And I get these errors:

CMake Error at cmake/modules/sysbuild_extensions.cmake:514 (message):
  CMake configure failed for Zephyr project: central_uart

  Location: /home/robertw/Software/Nordic/custom_service/central_uart
Call Stack (most recent call first):
  cmake/modules/sysbuild_images.cmake:20 (ExternalZephyrProject_Cmake)
  cmake/modules/sysbuild_default.cmake:20 (include)
  /home/robertw/ncs/v2.8.0/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:75 (include)
  /home/robertw/ncs/v2.8.0/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:92 (include_boilerplate)
  /home/robertw/ncs/v2.8.0/zephyr/share/sysbuild-package/cmake/SysbuildConfig.cmake:8 (include)
  template/CMakeLists.txt:10 (find_package)

If I remove that and add this:

&pinctrl {

uart0_default: uart0_default {

group1 {

psels = <NRF_PSEL(UART_TX, 0, 10)>,

<NRF_PSEL(UART_RTS, 0, 13)>;

};

group2 {

psels = <NRF_PSEL(UART_RX, 0, 09)>,

<NRF_PSEL(UART_CTS, 0, 15)>;

bias-pull-up;

};

};

};
I get these errors:

  Location: /home/robertw/Software/Nordic/custom_service/central_uart
Call Stack (most recent call first):
  cmake/modules/sysbuild_images.cmake:20 (ExternalZephyrProject_Cmake)
  cmake/modules/sysbuild_default.cmake:20 (include)
  /home/robertw/ncs/v2.8.0/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:75 (include)
  /home/robertw/ncs/v2.8.0/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:92 (include_boilerplate)
  /home/robertw/ncs/v2.8.0/zephyr/share/sysbuild-package/cmake/SysbuildConfig.cmake:8 (include)
  template/CMakeLists.txt:10 (find_package)

It tells me look at CMakeOutput.log

But that has the hilariously unhelpful:

The system is: Linux - 6.6.105-desktop-1.mga9 - x86_64

Line and nothing else in it. :-(

[Edvin]

Can you please upload your application, and state clearly what pins you want to use, and I can try to have a look?

Also, when you are pasting code snippets, it is easier to read (indentation is much better) if you use "insert"->"Code" here in DevZone.

BR,
Edvin