J-Link DLL error when flashing Ezurio BL54L15 via nRF54L-DK — SWD trace shows persistent ACK FAULT

[NandanS]

I'm trying to flash an external board using the Ezurio BL54L15 module, with the nRF54L15-DK as the debugger. SWD is connected via the DK's Debug Out header to the module's SWDIO and SWDCLK pins. Power and ground are supplied independently, and the SELECT pin on the SWD connector is grounded.

Flashing consistently fails with the following error:

Error: One or more batch tasks failed: * 1057720359: Device error: Failed to write DebugPort register 0x08: Unknown Error in J-Link DLL (error code = -1) (Generic)

To investigate, I captured the SWD trace — attached below — which shows persistent ACK FAULT responses during AccessPort Read CSW transactions. Key observations:

• Initial CSW reads return 0xFFFFFFFD (DbgSwEnable=1, Prot=0x7F, SPIDEN=1, Mode=0xF) with ACK OK and valid parity.
• DRW writes (e.g., 0x8827CFFD, 0x27CE4BFD) succeed early on.
• Midway through, Write CSW returns ACK WAIT, followed by ACK FAULT on subsequent Read CSW.
• Multiple Line Reset sequences occur, with repeated attempts to reconfigure CSW using Mode=0x1, 0x5, and 0xF.
• Eventually, all CSW reads fail with ACK FAULT, even after resets.

Would appreciate help with:

• Diagnosing the J-Link DLL error in this context
• Valid CSW mode values for nRF54L15 targets
• Conditions that trigger persistent ACK FAULT
• Recovery strategies after AP fault
• Any quirks specific to the Ezurio BL54L15 module or its debug interface
  
  

  Trace (Saleae Logic 2) is attached. Thanks in advance!

2656.SWD_capture.sal

0184.SWD_capture.csv

[Bendik Heiskel]

Hi,

What VDD voltage is the external board using?

The debug out connector on the nRF54L15DK is using a level shifter. The reference voltage for the level shifter is taken from pin 1 on the debug out connector(P19).

Are you supplying this pin with the VDD voltage from the external board?

 

Best regards,

Bendik

[NandanS]

The external board has it's own +3.3V source and supplies it to pin 1 of it's SWD connector.

[NandanS]

After obtaining a proper J-link module from Segger, I am able to program my custom board using the Segger J-link. My guess is that the NRF54L Devkit is not suitable for programming external boards.

Another similar ticket reflects the same and suggests that cable length can be an issue:
nRF54L DK can't program external MCU via SWD – J-Link error (Unknown Error in DLL)

I tested this by purchasing and using a shorter SWD cable (2.5cm) between the external board and nRF54L15-DK (Debug Out). With this configuration, I was also able to program my custom external board.

However, given that this cable-length constraint makes the table setup very cluttered, I will instead be using the Segger J-link going forward.