Arm Mbed and Pelion Device Management support forum

TransferTimeoutError: STLink error (20): DP wait

Hi all,
when I start debugging in Mbed Studio ( Version 0.7.0) with Nucleo-L476RG I’ll get following error

File “c:\Users\ste\AppData\Local\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\utility\”, line 201, in invoke
resultSequence = call()
File “c:\Users\ste\AppData\Local\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\coresight\”, line 121, in power_up_debug
self.write_reg(DP_SELECT, 0)
File “c:\Users\ste\AppData\Local\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\coresight\”, line 117, in write_reg
self.write_dp(addr, data)
File “c:\Users\ste\AppData\Local\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\coresight\”, line 243, in write_dp, data)
File “c:\Users\ste\AppData\Local\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\probe\”, line 168, in write_dp
result = self._link.write_dap_register(STLink.DP_PORT, addr, data)
File “c:\Users\ste\AppData\Local\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\probe\stlink\”, line 425, in write_dap_register
File “c:\Users\ste\AppData\Local\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\probe\stlink\”, line 292, in _check_status
raise self._ERROR_CLASSESstatus
TransferTimeoutError: STLink error (20): DP wait

Is there any solution for that?

ARM Compiler 6.11 up to date
Clangd 8.0.0 up to date
Example projects 1.2.0 up to date
GDB client 6-2017-q2-update up to date
Git 2.19.2 up to date
Mbed Library Cache 1.2.0 up to date
Python tools 0.2.0 up to date
Debug Packs 1.0.3 up to date

OS: Windows 10

Thank you

Hi Stepan,

We’ve addressed this error and it’s fixed, the next release will fix this issue.

Hi @s_urban ,

Thank you for reporting this issue. As @sayhuthut mentioned this issue should be resolved in the newest version of Mbed Studio. You can either download Mbed Studio 0.8.0 directly from or use auto-update mechanism inside Mbed Studio 0.7.0.

Arek - Mbed Studio team

I am using 0.8.0 with the Nucleo-L476RG board. I see the same kind of error when running the “example mbed-os-example-blinky”
I’m using first time install of the Mbed Studio. Here is the stack:

[Switching to Thread 536877572]
Thread 2 “main” hit Breakpoint 1, main () at ./main.cpp:18
18 SystemReport sys_state( SLEEP_TIME * PRINT_AFTER_N_LOOPS /* Loop delay time in ms */);
0017012:WARNING:gdbserver:Exception while target was running: STLink error (20): DP wait
Traceback (most recent call last):
File “c:\ProgramData\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\gdbserver\”, line 739, in resume
File “c:\ProgramData\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\coresight\”, line 652, in halt
self.write_memory(CortexM.DHCSR, CortexM.DBGKEY | CortexM.C_DEBUGEN | CortexM.C_HALT)
File “c:\ProgramData\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\coresight\”, line 611, in write_memory
self.ap.write_memory(addr, value, transfer_size)
File “c:\ProgramData\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\probe\”, line 222, in write_memory
self._link.write_mem32(addr, conversion.u32le_list_to_byte_list([data]), self._apsel)
File “c:\ProgramData\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\probe\stlink\”, line 378, in write_mem32
self._write_mem(addr, data, Commands.JTAG_WRITEMEM_32BIT, self.MAXIMUM_TRANSFER_SIZE, apsel)
File “c:\ProgramData\Mbed Studio\mbed-studio-tools\python\lib\site-packages\pyocd\probe\stlink\”, line 366, in _write_mem
raise exc
TransferFaultError: SWD/JTAG Transfer Fault @ 0xe000edf0-0xe000edf7