nrf54l15 Cannot use SWD interface

[Yeonjun Lee]

I recently designed and ordered custom nrf54l15 board but i cannot connect it through JLINK

It randomly throughs error like

```

(base) [ 10:18오후 ] [ yeonfish@Yeonjunui-MacBookPro:~ ]
$ nrfutil device recover
 Failed to recover 801044181, Device error: Setting the debug port SELECT register failed while powering up sys and debug regError: One or more recover tasks failed:
* 801044181: Device error: Setting the debug port SELECT register failed while powering up sys and debug regions (Unknown error in the J-Link DLL (error code =-1)).

This might indicate that the debug port is unavailable. Double-check device power supply and SWD lines, and ensure that SWD is not disabled (Generic)

(base) [ 10:18오후 ] [ yeonfish@Yeonjunui-MacBookPro:~ ]
$ nrfutil device recover
 Failed to recover 801044181, Device error: Failed to read DebugPort register 0x04: Unknown error in the J-Link DLL (error coError: One or more recover tasks failed:
* 801044181: Device error: Failed to read DebugPort register 0x04: Unknown error in the J-Link DLL (error code =-1) (Generic)

```

```

TF9FD58C0 005:619.607 Device "NRF54L15_M33" selected.
TF9FD58C0 005:620.746 - 2.079ms returns 0x00
TF9FD58C0 005:620.772 JLINK_EnableLog(...)
TF9FD58C0 005:620.779 - 0.008ms
TF9FD58C0 005:620.993 JLINK_GetEmuCaps()
TF9FD58C0 005:621.020 - 0.026ms returns 0xB8EA5A33
TF9FD58C0 005:621.035 JLINK_TIF_GetAvailable(...)
TF9FD58C0 005:621.214 - 0.178ms
TF9FD58C0 005:621.234 JLINK_TIF_Select(JLINKARM_TIF_SWD)
TF9FD58C0 005:621.884 - 0.650ms returns 0x00
TF9FD58C0 005:621.901 JLINK_IsConnected()
TF9FD58C0 005:621.909 - 0.008ms returns FALSE
TF9FD58C0 005:621.917 JLINK_SetSpeed(5000)
TF9FD58C0 005:622.061 - 0.144ms
TF9FD58C0 005:622.070 JLINK_Connect()
TF9FD58C0 005:622.560 ConfigTargetSettings() start
TF9FD58C0 005:622.570 J-Link Script File: Executing ConfigTargetSettings()
TF9FD58C0 005:622.590 ConfigTargetSettings() end - Took 17us
TF9FD58C0 005:622.599 InitTarget() start
TF9FD58C0 005:622.605 J-Link Script File: Executing InitTarget()
TF9FD58C0 005:625.792 InitTarget() end - Took 3.18ms
TF9FD58C0 005:626.736 Found SW-DP with ID 0x6BA02477
TF9FD58C0 005:628.365 Failed to power up DAP
TF9FD58C0 005:680.817 ConfigTargetSettings() start
TF9FD58C0 005:680.940 J-Link Script File: Executing ConfigTargetSettings()
TF9FD58C0 005:680.953 ConfigTargetSettings() end - Took 9us
TF9FD58C0 005:681.040 InitTarget() start
TF9FD58C0 005:681.055 J-Link Script File: Executing InitTarget()
TF9FD58C0 005:682.630 InitTarget() end - Took 1.53ms
TF9FD58C0 005:788.991 Failed to attach to CPU. Trying connect under reset.
TF9FD58C0 005:790.139 Found SW-DP with ID 0x6BA02477
TF9FD58C0 005:790.720 SWD speed too high. Reduced from 4000 kHz to 2700 kHz for stability
TF9FD58C0 005:791.796 Failed to power up DAP
TF9FD58C0 005:791.837 - 169.767ms returns 0xFFFFFEFB
TF9FD58C0 011:529.226 JLINK_IsOpen()
TF9FD58C0 011:529.294 - 0.068ms returns 0x01
TF9FD58C0 011:534.575 JLINK_Close()
TF9FD58C0 011:543.752 - 9.176ms
TF9FD58C0 011:543.810
TF9FD58C0 011:543.820 Closed

```

JLink connection failed by DAP power failure on clock speed 10000KHz

But i lower the speed, error message changes like this

```

(base) [ 9:29오후 ] [ yeonfish@Yeonjunui-MacBookPro:~ ]
$ JLinkExe -device NRF54L15_M33 -if SWD -speed 2000 -autoconnect 0 -log /tmp/jlink.log
SEGGER J-Link Commander V9.24a (Compiled Mar 5 2026 11:00:38)
DLL version V9.24a, compiled Mar 5 2026 10:59:55

Connecting to J-Link via USB...O.K.
Firmware: J-Link EDU Mini V1 compiled Feb 2 2026 08:45:19
Hardware version: V1.00
J-Link uptime (since boot): 0d 00h 05m 23s
S/N: 801044181
License(s): FlashBP, GDB
USB speed mode: Full speed (12 MBit/s)
VTref=3.197V

Type "connect" to establish a target connection, '?' for help
J-Link>r
Target connection not established yet but required for command.
Device "NRF54L15_M33" selected.

Connecting to target via SWD
ConfigTargetSettings() start
ConfigTargetSettings() end - Took 17us
InitTarget() start
InitTarget() end - Took 102ms
Failed to attach to CPU. Trying connect under reset.
ConfigTargetSettings() start
ConfigTargetSettings() end - Took 14us
InitTarget() start
InitTarget() end - Took 4.26ms
Error occurred: Could not connect to the target device.
For troubleshooting steps visit: https://kb.segger.com/J-Link_Troubleshooting

```

I did check the SWD cable length is <10cm and nRESET line voltage level. All of them was ok.

nRESET was 3.3v without pull up.

And i did tried to cut the nRESET line leads to jlink. and it did not worked either.

On PCBA, I forgot to connect the pin 10 to vdd. There was only a decoupling. So i soldered it to the vdd line. Solder was clean.

This is circuit board

And i soldered like this

[Yeonjun Lee]

UPDATE: I see the voltage around 1v on decoupling pins. looks fine. And i tried without oled attached. It did not work

[Yeonjun Lee]

UPDATE: 

Found weird volatge peaking on DECA.

Trying to feature out the root cause. Maybe due to putting C21 on bottom size connected with via...?

Peaking rate is approximately 51Khz

UPDATE: Tried to cut the line that connects DECA and DECRF. DECRF was fine. DECA still makes some speak graphs

UPDATE: removed C21. and.. it worked... why......?

[Szabolcs]

Hi,

what you are seeing here is probably L3 and C21 resonating (forming an LC tank circuit). On your schematic, you are missing a 2.2 uF capacitor on the DECA/DECRF net, that could probably explain the problem.

On the PCB, a VDD trace is overlapping with the GND plane, where you made the rework. Did you cut this trace? (edit: I see that the actual PCB is different, as the capacitor is rotated.)

Did you verify there is no short circuit, and all supply voltage levels are correct? 1V sounds a bit low, the internal voltage should be around 1.2V. What I see on the scope, is that the regulator fails to start correctly and tries again periodically.

[Yeonjun Lee]

Oh it was not applied on pcba. I did it after i got a board. And board works fine with 2.2uF Thanks!