NRF51-DK Problem debugging mbed-os-example-blinky

Hi All,

I am new to MBED though I have used CMSIS-RTOS-RTX on a project back in the day when MBED was just getting started. I am using Mbed Studio v 1.0 downloaded in the last few days.

I have a new NRF51-DK board that I have updated the DAPLink firmware to version 0253 using the process described here, without apparent issue.

The MBED OS 5 version of the mbed-os-example-blinky program builds for without issue and I can download it to the target cpu on the NRF51-DK using the button (having the play icon) in Mbed Studio. This works fine and I can change the LED flash rate by setting BLINKING_RATE_MS to different values in main.cpp and seeing the results.

My problem arises when I try to debug the program (still in , obviously). The program builds and compiles OK and attaches to the debugger but I get the following error

05028:INFO:gdbserver:RTX5 loaded successfully
Attached to debugger on port 50000
0005170:WARNING:flash_builder:Chip erase used when flash address 0x1b000 is not the same as flash start 0x0
0007739:INFO:loader:Erased chip, programmed 67584 bytes (66 pages), skipped 0 bytes (0 pages) at 25.17 kB/s
Image loaded: c:\Users\Mike-remote\Mbed Programs\mbed-os-example-blinky-5-v2\BUILD\NRF51_DK\GCC_ARM\mbed-os-example-blinky-5-v2.elf
Note: automatically using hardware breakpoints for read-only addresses.
[New Thread 2]
[New Thread 536887008]
[New Thread 536886672]
[New Thread 536886940]
2 “Handler mode” received signal SIGSEGV, Segmentation fault.
[Switching to Thread 2]
0xfffffffe in ?? ()

Can anyone give me any ideas what is going wrong here and what I need to do to fix the problem ? The exact same .hex output file runs OK when I download it to the target by copying to the DAPLINK disk in Windows.


Just to say that in the above

“builds for” should read “builds for Build profile\Debug”


“(still in , obviously)” should read “(still in Build profile\Debug, obviously)”

Hi Mike,

Thank you for reporting this issue. Mbed Studio is using pyOCD to flash and debug program on the board. I believe that it may have an issue with debugging this particular board. We will investigate it further.

Arek - Mbed Studio team