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]

Hello, 

You can enable the Logging module of Zephyr by adding #include <logging/log.h> to e.g. main.c and the following lines to prj.conf

# Configure logger
CONFIG_LOG=y
CONFIG_USE_SEGGER_RTT=n
CONFIG_LOG_BACKEND_UART=y
CONFIG_LOG_DEFAULT_LEVEL=4

There are four severity levels available in the system: error, warning, info and debug.

In your main.c add the following lines just before your main loop.

#define LOG_MODULE_NAME app
LOG_MODULE_REGISTER(LOG_MODULE_NAME);

In you application you should be able to replace e.g the printk() with LOG_INF() for debug/logging information.  

This should provide you with the following output

Let me know how that works for you

[JonnyMN]

This did not fix my problem, unfortunately.

Just to clear up any confusion, this is what I'm seeing as the device starts up:

*** 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
*** Booting Zephyr OS build v2.7.99-ncs1-1  ***
Flash regions..Domain..Permissions
00 03 0x00000 0x20000 .Secure..rwxl
04 31 0x20000 0x100000 .Non-Secure.rwxl

Non-secure callable region 0 placed in flash region 3 with size 32.

SRAM region..Domain..Permissions
00 03 0x00000 0x08000 .Secure..rwxl
04 31 0x08000 0x40000 .Non-Secure.rwxl

Peripheral..Domain..Status
00 NRF_P0               Non-Secure.OK
01 NRF_CLOCK            Non-Secure.OK
02 NRF_RTC0             Non-Secure.OK
03 NRF_RTC1             Non-Secure.OK
04 NRF_NVMC             Non-Secure.OK
05 NRF_UARTE1           Non-Secure.OK
06 NRF_UARTE2           Secure..SKIP
07 NRF_TWIM2            Non-Secure.OK
08 NRF_SPIM3            Non-Secure.OK
09 NRF_TIMER0           Non-Secure.OK
10 NRF_TIMER1           Non-Secure.OK
11 NRF_TIMER2           Non-Secure.OK
12 NRF_SAADC            Non-Secure.OK
13 NRF_PWM0             Non-Secure.OK
14 NRF_PWM1             Non-Secure.OK
15 NRF_PWM2             Non-Secure.OK
16 NRF_PWM3             Non-Secure.OK
17 NRF_WDT              Non-Secure.OK
18 NRF_IPC              Non-Secure.OK
19 NRF_VMC              Non-Secure.OK
20 NRF_FPU              Non-Secure.OK
21 NRF_EGU0             Non-Secure.OK
22 NRF_EGU1             Non-Secure.OK
23 NRF_EGU2             Non-Secure.OK
24 NRF_EGU3             Non-Secure.OK
25 NRF_EGU4             Non-Secure.OK
26 NRF_EGU5             Non-Secure.OK
27 NRF_DPPIC            Non-Secure.OK
28 NRF_REGULATORS       Non-Secure.OK
29 NRF_PDM              Non-Secure.OK
30 NRF_I2S              Non-Secure.OK
31 NRF_GPIOTE1          Non-Secure.OK

SPM: NS image at 0x20200
SPM: NS MSP at 0x20015338
SPM: NS reset vector at 0x25ec1
SPM: prepare to jump to Non-Secure image.
*** Booting Zephyr OS build v2.7.99-ncs1-1  ***

I don't want any of these messages on my UART 0 port. I only want messages that I send using printf() function in my code only. 

Here is what I want to see during device startup:

// nothing, but I had to put something here since the code block wont let me post without any text.

[Øyvind]

Ok, I see. You could use e.g. RTT for logs. See RTT chapter in Logging in nRF Connect SDK. To turn off the "boot banner" you can add CONFIG_BOOT_BANNER=n. 

Also, please see the Zephyr Logging API. 

[JonnyMN]

I added the following to my prj.conf file:

CONFIG_BOOT_BANNER=n

CONFIG_LOG=y
CONFIG_LOG_MODE_MINIMAL=y
CONFIG_LOG_BACKEND_UART=n
CONFIG_USE_SEGGER_RTT=y
CONFIG_LOG_BACKEND_RTT=y

But it had no effect. Boot banner and all the Zephyr start-up messages are still present on UART0.

[JonnyMN]

Are there any updates on this? Again, I want to silence all outputs to the UART0, and only transmit data I send out via a printf() function.

[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!