Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

rbence — Sat, 30 Aug 2025 12:56:38 GMT

Thank you very much.

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

zener — Fri, 29 Aug 2025 08:54:03 GMT

Yes, at least I have the location of the startup code in nrfx.git/mdk passed in my CFLAGS -I option. Which results in:

Creating directory build
[CC] main.c --> build/main.o
Creating directory build/cmsis
[AS] /home/myuser/projects/nordic/nrfx.git/mdk/gcc_startup_nrf5340_application.S --> build/cmsis/startup.o
[CC] /home/myuser/projects/nordic/nrfx.git/mdk/system_nrf5340_application.c --> build/cmsis/system.o

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

rbence — Sat, 23 Aug 2025 11:07:38 GMT

Hi,

I'm also trying to create a bare metal app. Did you use the contents of the nrfx/mdk folder? I mean the gcc_startup_nrf5340_application.s and system_nrf5340_application.c and for linker script the nrf5340_xxaa_application.ld?
Thank you.

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

zener — Tue, 19 Sep 2023 21:34:47 GMT

One more note: It's also possible to use the JLinkRTTLogger and the output will end up in /tmp/stdout.log

/opt/SEGGER/JLink_Linux_V792e_x86_64/JLinkRTTLogger -SelectEmuBySN 1050169338 -Device NRF5340_XXAA_APP -If swd -Speed 4000 -RTTAddress auto-detection -RTTChannel 0 /tmp/stdout.log

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

zener — Tue, 19 Sep 2023 21:08:39 GMT

A short note if somebody else would end up here due to search for the same.

It was quite simple to retarget the stdio using the segger provided files. Basically all I had to do was to compile and link SEGGER_RTT.c and SEGGER_RTT_Syscalls_GCC.c and then observe the output on port 19021:

$ nc localhost 19021
SEGGER J-Link V7.92e - Real time terminal output
SEGGER J-Link (unknown) V1.0, SN=1050169338
Process: JLinkGDBServerCLExe
Hello devkit
git sha1 09d8694e4695f6c44a5b4d307587e49dc3b5bfeb, branch rtt-output-support

By running this:

#include <stdio.h>

int main()
{
printf("Hello devkit\n");
#ifdef GIT_SHA1
printf("git sha1 %s, branch %s\n",GIT_SHA1,GIT_BRANCH);
#endif
...

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

Vidar Berg — Tue, 19 Sep 2023 12:12:06 GMT

Unfortunately, we do not have a bare metal retarget implementation that works with 'nrfx'. In the SDK, we use the Zephyr serial drivers for UART. For your UART send function, you can use the UARTE driver from the nrfx repo at https://github.com/NordicSemiconductor/nrfx/tree/master

For RTT, you need to include the RTT source code in your project. The source code is included in the JLink installation and can be found in <jlink install dir>/Samples/RTT

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

zener — Tue, 19 Sep 2023 11:18:19 GMT

Thanks! Is there such a retargeting library available already, e.g. on top of the nrfx uart or something using the segger serial port(s)?

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

Vidar Berg — Tue, 19 Sep 2023 11:01:01 GMT

Hello,

Yes, you must retarget printf() to redirect the output to UART or RTT.

#include <stdio.h>

// low-level function used by printf() to write output data
int _write(int file, const char * p_char, int len)
{
    // TODO: send printf() bytes to UARTE
    
    return len;
}

Best regards,

Vidar

Edit: This stackoverflow post (https://stackoverflow.com/questions/55014043/how-do-you-implement-printf-in-gcc-from-newlib) covers this topic in more detail.

RE: Bare metal gcc nRF5340 Audio DK printf/stdout uart output?

zener — Tue, 19 Sep 2023 10:46:31 GMT

I guess this boils down to if there is a uart or segger driver for the gcc stdio library to enable output of printf/stdout in the nrfx repository or even using CMSIS or similar, or is using explicit uart calls the only option?