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

[zener]

I have made a bare metal app using gcc on Linux for the nRF5340 Audio DK. I'm able to start the segger gdb server, load the code, set a breakpoint, run and watch the LED's flash as expected.

However, I don't observe any printf/stdout output from the app. The problem is I don't know where to expect the output using the default configuration?

This is the code:

#include "nrfx.h"
#include <stdio.h>

int main()
{
printf("Hello devkit\n");
// then the NRF_P1->DIRSET etc which seem to be working
...

The compiler/linker does not complain so there is some printf support even though I don't know if any uart initialization or other code is required to support it?

When I plug in the board four /dev/tty devices are being created. I don't observe the printf output on any of these or in gdb.

nrfutil device list --traits seggerUsb returns:

ports /dev/ttyACM0, vcom: 0
/dev/ttyACM1, vcom: 1
/dev/ttyUSB5, vcom: 1
/dev/ttyUSB4, vcom: 2
traits jlink, seggerUsb, serialPorts, usb

Using these parameters:
set line /dev/ttyXXXX
set speed 115200
set carrier-watch off
set flow-control none

Where XXXX is one of the four ports given by nrfutil device list above.

There are two UART ports on the board itself. I have not checked these. Can I expect to observe the printf/stdout if I connect any of these?

I don't get any output in gdb, nor at the tcp/ip port given by the gdb server

SEGGER J-Link GDB Server V7.92e Command Line Version

JLinkARM.dll V7.92e (DLL compiled Sep 13 2023 15:44:25)

Command line: -nogui -nosilent -port 2131 -if swd -device NRF5340_XXAA_APP
-----GDB Server start settings-----
GDBInit file: none
GDB Server Listening port: 2131
SWO raw output listening port: 2332
Terminal I/O port: 2333
Accept remote connection: yes
Generate logfile: off
Verify download: off
Init regs on start: off
Silent mode: off
Single run mode: off
Target connection timeout: 0 ms
------J-Link related settings------
J-Link Host interface: USB
J-Link script: none
J-Link settings file: none
------Target related settings------
Target device: NRF5340_XXAA_APP
Target device parameters: none
Target interface: SWD
Target interface speed: 4000kHz
Target endian: little

Connecting to J-Link...
J-Link is connected.
Firmware: J-Link OB-nRF5340-NordicSemi compiled Aug 8 2023 16:16:56
Hardware: V1.00
S/N: 1050169338
Feature(s): RDI, FlashBP, FlashDL, JFlash, GDB
Checking target voltage...
Target voltage: 3.30 V
Listening on TCP/IP port 2131
Connecting to target...
Halting core...
Core security extensions: Implemented
Connected to target
Waiting for GDB connection...

$ nc localhost 2333
SEGGER J-Link GDB Server V7.92e - Terminal output channel

Then nothing here either. Where would I expect to observe the printf/stdout?

Will JLinkRTTLogger be able to capture the stdout, if yes how can this be done for the nRF5340 Audio DK?

[zener]

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?

[Vidar Berg]

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.

[zener]

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)?

[Vidar Berg]

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

[zener]

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
...