PyOCD cannot connect to ARM SWDAP probe with STM32F767ZI target.

I'm an OpenOCD defector, making my first steps in the world of PyOCD. But I can't get my microcontroller board to work. Please help.

1. Setup

1.1 Probe

I've got the official SWDAP probe from ARM (bought at L-Tek https://www.l-tek.com/web-shop/l-tek-swdap-interface/):

I flashed the latest 0254_lpc11u35_0x0000.bin DAPLink firmware to my probe (see https://github.com/ARMmbed/DAPLink/releases). So the SWDAP probe is up-to-date.
 

1.2 Microcontroller board

I've got a self-made board with an STM32F767ZI microcontroller. The board is powered from a USB-micro connector (which is for power only, no data). I've got a 10-pin Cortex Debug connector for SWD flashing and debugging:

I connect the SWDAP probe to my board with the ribbon cable that was shipped with my SWDAP probe.
 

1.3 Desktop

My desktop computer runs 64-bit Windows 10 with the latest Python 3.8. I just installed PyOCD today. I did not use pip install but I cloned the PyOCD GitHub and issued the command:

$ python setup.py install

So I got the latest development version of PyOCD. I also put the libusb dll next to python.exe in my Python 3.8 installation folder, as requested in the installation instructions.
 

2. Running PyOCD

2.1 Find probe

First I check if PyOCD finds the probe:

$ pyocd list

      #   Probe              Unique ID
    ---------------------------------------------------------------------------
      0   ARM CMSIS-DAP v1   000000800a0c882800000000000000000000000097969902

So that seems allright.
 

2.2 Download pack for STM32F767ZI

Unfortunately the STM32F767ZI microcontroller is not in the builtin targets. I issued the command pyocd list --targets and couldn't find it. So I download the corresponding pack Keil.STM32F7xx_DFP.2.12.0.pack from http://www.keil.com/dd2/pack/.
 

2.3 Start GDB server

I issue the following command to make PyOCD connect to my microcontroller and start a GDB server:

pyocd gdbserver --target=stm32f767zi --pack="C:/ ... /packs/Keil.STM32F7xx_DFP.2.12.0.pack"

PyOCD immediately prints out:

    0000874:INFO:board:Target type is stm32f767zi

So I believe it's time to start the GDB client.
 

2.4 Start GDB client

I open another Windows console, and issue the command:

$ arm-none-eabi-gdb

    (gdb) target remote localhost:3333
    localhost:3333: No connection could be made because the target machine actively refused it.
    (gdb)

I've got no luck here.

After a few seconds, I notice the following output in the first console (the one running PyOCD):

# Note: this is output from the first console - the one running PyOCD
# -------------------------------------------------------------------

0020877:CRITICAL:__main__:Read timed out
Traceback (most recent call last):
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\probe\cmsis_dap_probe.py", line 151, in open
    self._link.open()
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\probe\pydapaccess\dap_access_cmsis_dap.py", line 580, in open
    self._packet_count = self._protocol.dap_info(self.ID.MAX_PACKET_COUNT)
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\probe\pydapaccess\cmsis_dap_core.py", line 135, in dap_info
    resp = self.interface.read()
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\probe\pydapaccess\interface\pywinusb_backend.py", line 162, in read
    raise DAPAccessIntf.DeviceError("Read timed out")
pyocd.probe.pydapaccess.dap_access_api.DAPAccessIntf.DeviceError: Read timed out

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\__main__.py", line 343, in run
    self._COMMANDS[self._args.cmd](self)
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\__main__.py", line 571, in do_gdbserver
    with session:
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\core\session.py", line 291, in __enter__
    self.open()
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\core\session.py", line 367, in open
    self._probe.open()
  File "C:\Python38\lib\site-packages\pyocd-0.23.1.dev10-py3.8.egg\pyocd\probe\cmsis_dap_probe.py", line 163, in open
    six.raise_from(self._convert_exception(exc), exc)
  File "<string>", line 3, in raise_from
pyocd.core.exceptions.ProbeError: Read timed out

 

3. Conclusion

I tried several times to get the connection working. But I keep getting the same 0020877:CRITICAL:__main__:Read timed out error (with the whole traceback after it). In fact, it doesn't matter if I do or don't start the GDB client. PyOCD throws this error after perhaps 10 or 20 seconds - no matter what I do.

Note:
The problem is not related to my board. I've flashed this board successfully with other probes using OpenOCD. So the hardware is OK.

Reproduce the reported command with the ARM CMSIS-DAP v1 probe and STM32F767ZI pack, then trace the timeout through pyocd/probe/cmsis_dap_probe.py, pyocd/probe/pydapaccess/dap_access_cmsis_dap.py, and pywinusb_backend.py. Done means identifying and fixing the cause so gdbserver starts without the reported Read timed out error.