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Hang Starting nRF52840 Feather Express Under Segger Debugger

I’m trying to use the Segger debugger on an adafruit_feather_nrf52840 board under Platformio. I’m not sure if this and an Adafruit problem or a Platformio problem so I also posted it on the Adafruit forum.

Even in athe most trivial project the debugger repeatedly stops during initialization at NVIC_SetPriority and will not proceed beyond this point. This is my project:

#include <Arduino.h>
void setup()
  Serial.println("setup test");
  // put your setup code here, to run once:
void loop() {
  // put your main code here, to run repeatedly:
  Serial.println("loop test");


platform = nordicnrf52
board = adafruit_feather_nrf52840
framework = arduino

To replicate the problem:

  • Click the “Run and Debug” button on the left in Platformio to enter the debugging panel.

  • Click the “Start Debugging” button at the top.

  • After a moment the program stops at main() in main.cpp:
    // \brief Main entry point of Arduino application
    int main( void )

  • Press F5 to continue. The program halts in core_cm4.h here:

    __STATIC_INLINE void NVIC_SetPriority(IRQn_Type IRQn, uint32_t priority)
    if ((int32_t)(IRQn) < 0)
    SCB->SHP[(((uint32_t)(int32_t)IRQn) & 0xFUL)-4UL] = (uint8_t)((priority << (8U - __NVIC_PRIO_BITS)) & (uint32_t)0xFFUL);
    NVIC->IP[((uint32_t)(int32_t)IRQn)] = (uint8_t)((priority << (8U - __NVIC_PRIO_BITS)) & (uint32_t)0xFFUL); //Halts here

  • Now every time F5 is pressed to continue the program halts in the same place.

  • It’s impossible to debug using the hardware debugger.

However if the program is started without the debugger it runs normally and prints its messages to the serial monitor.

I’m running on a Macbook pro 2015 with the the Feather board plugged into one USB port and the Segger debugger plugged into the other port.

How can I avoid the halting?

Well that’s wrong.

  • Do you get a GDB console when you go into the Debug console? What’s the result of info breakpoints?
  • A screenshot of VSCode, especially regarding the section “Breakpoints”?

Below is the content of the debug console after the following actions:

Build, proceed at main(), halt at NVIC_SetPriority, and an attempt to proceed and another halt at halt at NVIC_SetPriority
At the end of the listing is the reply to “info breakpoints” (says there are no breakpoints).
The PIO Breakpoints section is empty.

Debug console output:

Processing adafruit_feather_nrf52840 (platform: nordicnrf52; board: adafruit_feather_nrf52840; framework: arduino)
Verbose mode can be enabled via `-v, --verbose` option
PLATFORM: Nordic nRF52 4.0.0 > Adafruit Feather nRF52840 Express
HARDWARE: NRF52840 64MHz, 243KB RAM, 796KB Flash
DEBUG: Current (jlink) External (jlink, stlink)
 - framework-arduinoadafruitnrf52 1.1405.191023 (14.5)
 - tool-sreccat 1.164.0 (1.64)
 - toolchain-gccarmnoneeabi 1.70201.0 (7.2.1)
LDF: Library Dependency Finder -> http:/ /
LDF Modes: Finder ~ chain, Compatibility ~ soft
Found 12 compatible libraries
Scanning dependencies...
No dependencies
Building in debug mode
Checking size .pio/build/adafruit_feather_nrf52840/firmware.elf
Advanced Memory Usage is available via "PlatformIO Home > Project Inspect"
RAM:   [          ]   2.4% (used 5976 bytes from 248832 bytes)
Flash: [          ]   4.2% (used 34164 bytes from 815104 bytes)
========================= [SUCCESS] Took 1.16 seconds =========================
SEGGER J-Link GDB Server V6.52 Command Line Version
JLinkARM.dll V6.52 (DLL compiled Sep 27 2019 17:51:01)

Command line: -singlerun -if SWD -select USB -device nRF52840_xxAA -port 2331
-----GDB Server start settings-----
GDBInit file:                  none
GDB Server Listening port:     2331
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:               on
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:                 nRF52840_xxAA
Target interface:              SWD
Target interface speed:        4000kHz
Target endian:                 little
Connecting to J-Link...
Reading symbols from /Users/chrisgr/Documents/PlatformIO/Projects/Test Jtag/.pio/build/adafruit_feather_nrf52840/firmware.elf...
PlatformIO Unified Debugger -> http:/ /
PlatformIO: debug_tool = jlink
PlatformIO: Initializing remote target...
J-Link is connected.
Firmware: J-Link V10 compiled Aug 27 2019 09:24:55
Hardware: V10.10
S/N: 260117951
Feature(s): FlashBP, GDB
Checking target voltage...
Target voltage: 3.31 V
Listening on TCP/IP port 2331
Connecting to target...
Connected to target
Waiting for GDB connection...
Connected to
Reading all registers
Read 4 bytes @ address 0x0002A04C (Data = 0x3D04F84D)
0x0002a04c in PendSV_Handler () at /Users/chrisgr/.platformio/packages/framework-arduinoadafruitnrf52/cores/nRF5/freertos/portable/GCC/nrf52/port.c:104
104    __asm volatile
Received monitor command: clrbp
Received monitor command: speed auto
Select auto target interface speed (2000 kHz)
Select auto target interface speed (2000 kHz)
Received monitor command: reset
Resetting target
Resetting target
Received monitor command: halt
Halting target CPU...
...Target halted (PC = 0x00000A80)
Loading section .text, size 0x832c lma 0x26000
Downloading 16032 bytes @ address 0x00026000
Downloading 15984 bytes @ address 0x00029EA0
Downloading 1564 bytes @ address 0x0002DD10
Loading section .ARM.exidx, size 0x8 lma 0x2e32c
Downloading 8 bytes @ address 0x0002E32C
Loading section .data, size 0x240 lma 0x2e334
Downloading 576 bytes @ address 0x0002E334
Start address 0x262b8, load size 34164
Writing register (PC = 0x   262b8)
Transfer rate: 16681 KB/sec, 6832 bytes/write.
Read 4 bytes @ address 0x000262B8 (Data = 0x4A0B490A)
Read 2 bytes @ address 0x000262B8 (Data = 0x490A)
Read 2 bytes @ address 0x00026334 (Data = 0xB500)
Temporary breakpoint 1 at 0x26334: file /Users/chrisgr/.platformio/packages/framework-arduinoadafruitnrf52/cores/nRF5/main.cpp, line 69.
PlatformIO: Initialization completed
PlatformIO: Resume the execution to `debug_init_break = tbreak main`
PlatformIO: More configuration options -> http:/ /
Setting breakpoint @ address 0x00026334, Size = 2, BPHandle = 0x0001
Starting target CPU...
...Breakpoint reached @ address 0x00026334
Reading all registers
Removing breakpoint @ address 0x00026334, Size = 2
Read 4 bytes @ address 0x00026334 (Data = 0xB083B500)

Temporary breakpoint 1, main () at /Users/chrisgr/.platformio/packages/framework-arduinoadafruitnrf52/cores/nRF5/main.cpp:69
69 {
info breakpoints
No breakpoints or watchpoints.

So when press F5 to forward execution and it halts at the “breakpoint”, what is printed in the GDB console? It should print something like “SIG<…> target halted / breakpoint hit”

It’s not a real breakpoint. It says:

Starting target CPU…
…Target halted (DBGRQ, PC = 0x00000A64)
Reading all registers

received signal SIGTRAP, Trace/breakpoint trap.
Read 4 bytes @ address 0x00000A64 (Data = 0x471868DB)
Read 2 bytes @ address 0x00000A64 (Data = 0x68DB)
0x00000a64 in ?? ()
Read 4 bytes @ address 0x2003FFAC (Data = 0x21000010)
Reading 64 bytes @ address 0x2003FF80
Read 4 bytes @ address 0x00000760 (Data = 0x47085809)
Read 2 bytes @ address 0x00000760 (Data = 0x5809)
Read 4 bytes @ address 0x2003FFCC (Data = 0x61000000)
Reading 64 bytes @ address 0x2003FFC0
Read 4 bytes @ address 0x0002A486 (Data = 0xF8832240)
Read 4 bytes @ address 0x0002A4E2 (Data = 0xFF0BF7FD)
Read 4 bytes @ address 0x00029EDC (Data = 0x93014B0E)

Also the halt happens with both of the Adafruit Express nRF52480 boards that I have. It also remains if I reboot my Mac.

It’s like it’s hitting an error condition such as an erroneous instruction, invalid memory accessed or an infinite loop caused by changing interrupt priority. However it’s in the default code. I added nothing to this project.

Well it’s hit a debugrequest / SIGTRAP at an very low program counter value. Per memory map the flash starts at 0x00000000, so 0xA64 is just 2660 bytes into the firmware.

Can you start another debugging session, let it halt at the weird NVIC_SetPriority “breakpoint” and then execute these commands

i r
x/10i $pc

in the GDB debug terminal? What’s the output?

1 Like

Here is the complete output from those commands:

i r
r0             0x2003ffd0

r1             0x239a

r2             0x380

r3             0xe000e100

r4             0x2000439c

r5             0x0

r6             0x0

r7             0x0

r8             0x0

r9             0x0

r10            0x20030000

r11            0x0

r12            0x80

sp             0x2003ffd0

lr             0x2a321

pc             0x2a322
	0x2a322 <usb_hardware_init()+34>

xpsr           0x61000000

msp            0x2003ff90

psp            0x0

primask        0x0

basepri        0x0

faultmask      0x0

control        0x0

#0  0x00000a64 in ?? ()
#1  <signal handler called>
#2  0x00000760 in ?? ()
#3  <signal handler called>
#4  0x0002a322 in NVIC_SetPriority (priority=2, IRQn=USBD_IRQn) at /Users/chrisgr/.platformio/packages/framework-arduinoadafruitnrf52/cores/nRF5/cmsis/include/core_cm4.h:1706
#5  usb_hardware_init () at /Users/chrisgr/.platformio/packages/framework-arduinoadafruitnrf52/cores/nRF5/Adafruit_TinyUSB_Core/Adafruit_TinyUSB_Core.cpp:72
#6  0x0002a37e in Adafruit_TinyUSB_Core_init () at /Users/chrisgr/.platformio/packages/framework-arduinoadafruitnrf52/cores/nRF5/Adafruit_TinyUSB_Core/Adafruit_TinyUSB_Core.cpp:88
#7  0x00029d78 in main () at /Users/chrisgr/.platformio/packages/framework-arduinoadafruitnrf52/cores/nRF5/main.cpp:74
Read 4 bytes @ address 0x0002628E (Data = 0xF949F006)
Reading 64 bytes @ address 0x00026240
Read 4 bytes @ address 0x00026298 (Data = 0x20040000)
Read 4 bytes @ address 0x00026296 (Data = 0x00000008)
Read 4 bytes @ address 0x000262A4 (Data = 0x20003650)
Read 4 bytes @ address 0x000262AA (Data = 0x00002000)
x/10i $pc
Reading 64 bytes @ address 0x0002A300
=> 0x2a322 <usb_hardware_init()+34>:	movs	r2, #64	; 0x40
   0x2a324 <usb_hardware_init()+36>:	strb.w	r2, [r3, #807]	; 0x327
   0x2a328 <usb_hardware_init()+40>:	mov.w	r3, #1073741824	; 0x40000000
   0x2a32c <usb_hardware_init()+44>:	ldr.w	r4, [r3, #1080]	; 0x438
   0x2a330 <usb_hardware_init()+48>:	tst.w	r4, #1
   0x2a334 <usb_hardware_init()+52>:	bne.n	0x2a340 <usb_hardware_init()+64>
   0x2a336 <usb_hardware_init()+54>:	tst.w	r4, #2
   0x2a33a <usb_hardware_init()+58>:	bne.n	0x2a348 <usb_hardware_init()+72>
   0x2a33c <usb_hardware_init()+60>:	add	sp, #8
   0x2a33e <usb_hardware_init()+62>:	pop	{r4, pc}

btw - I was also seeing this same behavior (and same register dump that @chrisgr99 got). I’m on a NRF52840-DK with the Adafruit BSP. The failure seems to only occur if I try to ‘step-over’ the startup of the freertos scheduler (vTaskStartScheduler call in main.cpp of the adafruit arduino env).

If I don’t do that (and instead just let it run) everything seems to work okay and I can set breakpoints nicely in my app.

Thanks “geeksville”. What you are reporting certainly seems related to my problem. However in the firmware for the Feather nRF52840, Adafruit_TinyUSB_Core_init() - where the hang occurs - is called before vTaskStartScheduler(). I never get to stage of being able to not step over vTaskStartScheduler.

The hang occurs in Adafruit_TinyUSB_Core_init whether I step over it or not. It seems that the rtos is using an interrupt that conflicts with the debugger.

If it is useful for you, I’ve made a little workaround for this (it started happening to me again).

It is a little gdbinit script that adds a “restart” command. I run this after start and all is well for nicely hitting only the breakpoints I set. A side benefit, I can run “restart” if I want to restart my load without having to rebuild/flash.


# the jlink debugger seems to want a pause after reset before we tell it to start running
define restart
  monitor reset 
  shell sleep 1

And in my platformio.ini:

debug_tool = jlink
build_type = debug ; I'm debugging with ICE a lot now
debug_extra_cmds =
  source gdbinit
1 Like

Thanks! This does seem to solve the problem. Specifically this is what I did:
Create a .gdbinit file in my home directory containing:

 define restart
     monitor reset 
     shell sleep 1

Change platformio.ini file to be:

platform = nordicnrf52
board = adafruit_feather_nrf52840
framework = arduino
debug_tool = jlink
upload_protocol = jlink
build_type = debug 
debug_extra_cmds = source gdbinit

Then in Platformio (VSCode ), start the debugger. (Press the Run button in the Activity Bar and then the Run - PIO Debug button in the top left.)
When the program stops at the first line in main.cpp type “restart” in the debug console, hit enter.
The program starts running normally and hits the first real breakpoint, if any.

This works. Is it the correct way to do it?

1 Like

Had same issue and fix worked. Had to set a HOME environment variable in Windows.