Pyocd and Picoprobe unable to flash nRF51822

Question

PyOCD and Picoprobe will not flash an NRF51822, exiting with the error cannot read register r0 because core #0 is not halted.

Setup:
Windows 11
Python 3.9.6
PyOCD 0.33.1

Output from pyocd load firmware.hex --target nrf51822:

0001527 I Loading C:\Users\henri\NRF5x\NRF_LED\.pio\build\nrf51822_y5_mbug\firmware.hex [load_cmd]
0006932 C cannot read register r0 because core #0 is not halted [__main__]
Traceback (most recent call last):
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 259, in _enable_read_access
    self.flash.init(self.flash.Operation.VERIFY)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 250, in init
    raise FlashFailure('flash init timed out')
pyocd.core.exceptions.FlashFailure: flash init timed out

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\__main__.py", line 161, in run
    status = cmd.invoke()
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\subcommands\load_cmd.py", line 117, in invoke
    programmer.program(filename,
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\file_programmer.py", line 170, in program
    self._loader.commit()
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\loader.py", line 289, in commit
    perf = builder.program(chip_erase=chipErase,
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 490, in program
    sector_erase_count, page_program_time = self._compute_sector_erase_pages_and_weight(fast_verify)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 666, in _compute_sector_erase_pages_and_weight
    self._analyze_pages_with_crc32(fast_verify)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 645, in _analyze_pages_with_crc32
    self._enable_read_access()
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 262, in _enable_read_access
    self.flash.init(self.flash.Operation.ERASE)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 243, in init
    result = self._call_function_and_wait(self.flash_algo['pc_init'],
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 620, in _call_function_and_wait
    return self.wait_for_completion(timeout=timeout)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 616, in wait_for_completion
    return self.target.read_core_register('r0')
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\core\soc_target.py", line 218, in read_core_register
    return self.selected_core_or_raise.read_core_register(id)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 971, in read_core_register
    regValue = self.read_core_register_raw(reg_info.index)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 988, in read_core_register_raw
    vals = self.read_core_registers_raw([reg])
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 1009, in read_core_registers_raw
    return self._base_read_core_registers_raw(reg_list)
  File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 1023, in _base_read_core_registers_raw
    raise exceptions.CoreRegisterAccessError(
pyocd.core.exceptions.CoreRegisterAccessError: cannot read register r0 because core #0 is not halted

If more information is needed, don't hesitate to ask.

Answer 1

The error message indicates that the debugger is unable to communicate with the target device because the processor is not in a halted state or debugger is unable to halt the target CPU core before attempting to read a register.

1. Ensure that the target device is not already running. If the processor is already executing code, it will not be in a halted state and the debugger will not be able to communicate with it. Make sure that the target device is not executing code before you try to connect to it.
2. Ensure that the target device is supported by PyOCD and that you have the correct version of PyOCD installed. Check the PyOCD documentation to see if your device is supported and make sure that you have the correct version of PyOCD installed.
3. Check the connection between the target device and the debugger.
4. Reset the target.

Let me know if these steps worked for you.