nRF9160 Using UART with printf without all the debugging outputs from Zephyr

[JonnyMN]

How do I use printf to output my own messages, and read incoming data, from a UART port that is separate from Zephyr's debugging outputs? This UART line will be connected to another microcontroller for custom commands.

Here is what I have so far:

I want printf messages to be directed to the UART0 port for my program to talk to another device. I also want to be able to read the incoming messages.

Here is my code that works:

while (1) {
		char *s = console_getline();
		printf("Here's your string: %s\n", s);
	}

However, with the above code, I also get what appears to be debug outputs from Zephyr being put on my UART0 line. For example, when the device starts up, I get a bunch of debug messages, etc. Also, any printk() messages are also on UART0.

I thought I could get around this by modifying the kconfig settings (via kconfig or guiconfig). I tried disabling things like the kernal boot banner, logging, etc, but the settings are reset to the defaults after rebuilding the project.

[Øyvind]

My apologies for the late answer. Can you please add the following folder and file to your project folder: child_image/spm.conf

In this file, add the following line: CONFIG_SERIAL=n

Then, in your prj.conf add the following:

CONFIG_UART_CONSOLE=n

CONFIG_RTT_CONSOLE=y 

Let me know how that works for you. 

Kind regards,
Øyvind

[JonnyMN]

I tried your suggestion, but I'm still getting this output from UART0:

*** Booting Zephyr OS build v2.7.99-ncs1-1  ***
I: Starting bootloader
I: Primary image: magic=bad, swap_type=0x0, copy_done=0x2, image_ok=0x2
I: Secondary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
I: Boot source: none
I: Swap type: none
I: Bootloader chainload address offset: 0x10000
I: Jumping to the first image slot
+CEREG: 2,"4603","04EE0211",7
+CSCON: 1
+CEREG: 5,"4603","04EE0211",7,,,"11100000","11100000"
%XTIME: "0A","2250011293210A","01"
+CSCON: 0

Would it be easier to use UART1 or UART2 for a custom communication channel instead? Is there any documentation on how to do this? (When I've tried to use UART1 or UART2 be modifying the dts or kconfig files, the program crashes).

[tedhavelka]

Hello JonnyMN,

Did you resolve your custom communication channel issue?  I happened to be searching for answers to a related Nordic ncs and Zephyr RTOS question when I came across your post.

To your latest question about a month ago, from my research and experiments it is easier to a UART1 or UART2, something other than Zephyr's chosen UART for its kernel messages.

I ran into this matter about nine months ago, where I had both Zephyr messages and nRF9160 LTE modem diagnostics printing to UART0.  I wanted a serial port for a simple command line interface for development purposes, a little different than your needs.  The specific problem I faced was not in seeing some extra messages from the modem and RTOS, but that a part of aws_iot sample app code was consuming my typed input and passing that along to the modem.

As a first step toward message free UART, I enabled a second UART in device tree.  Then I added some boiler plate Zephyr macro code to be to say `static const struct device *cli_uart`.  This is a device pointer in one of my firmware modules to the second, app-dedicated UART.

With these in place I was yet not able to find a way to redirect printf() or printk() to my alternate UART.  In place of relying on printk() I ended up using something you may find useful, or it may be too limited.  I use Zephyr's lower lever uart_poll_in() and uart_poll_out() APIs.  Here are links to pertinent lines in an open source project of my own on github:

1075 # github.com/.../thread-simple-cli.c
1076 # github.com/.../thread-simple-cli.c
1077 # https://github.com/tedhavelka/kionix-driver-demo/blob/main/src/thread-simple-cli.c#L105

This code is a few months old and in need of clean up for sure.

With my Zephyr based project I've had ample microcontroller memory resources to enable printf() family functions of newlibc.  The effective replacement for printk() which I implement around uart_poll_out() is not equivalent to printk() in its signature.  My function is very simple and just takes a constant string type.  As you likely know, printk() and printf() are variadic and allow for fancier string formatting.  You can still use my solution in conjunction with snprintf() to build your messages, then pass the completed string to the wrapper I mentioned.

Alternately if you are very memory constrained and can build your messages without printf() functions, you can leverage the above code at the cost of a smaller memory footprint in your project.

Not sure whether this will help, but I try to answer a question here every few times I search and get answers from the Devzone community.  Hope this can help.

- Ted

[JonnyMN]

Hi tedhavelka,

Thank you for the details response! Unfortunately, I won't have time to try out your suggestions until later this year. But I will make note of this when development picks up again!