I am writing regarding an issue I am experiencing with two MDBT53V-1M modules (nRF5340) that I have assembled on custom PCBs. Both modules exhibit identical behavior, which suggests a factory configuration issue rather than an assembly problem.
SETUP
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Debugger: Nordic nRF5340-DK (onboard J-Link OB, S/N: 1050060761)
Software: SEGGER J-Link Commander V7.94e / nrfjprog 10.24.2
Connection: Standard 10-pin 1.27mm SWD cable, J2 (2x5 Cortex debug header) -> P19 on nRF5340-DK
Target power: 3.3V supplied independently
VTref measured: 3.300V
OBSERVED BEHAVIOR
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1. VTref = 3.300V is correctly detected by the debugger.
2. DK successfully switches to external target (InitTarget() takes ~32ms, confirming target detection).
3. LED behavior strongly suggests the MCU is running firmware and actively responding to SWD signals:
- LED0 (P1.12, cathode) and LED1 (P1.13, cathode), both with 220 Ohm series resistors to 3.3V, BLINK during SWD connect attempts, indicating the MCU is executing code and being reset by the debugger.
- Both LEDs remain ON (steady) when the SWDSelect switching sequence is sent, indicating the MCU has detected and is actively responding to the SWD line reset sequence.
This rules out a simple power or assembly issue — the MCU is clearly alive and reacting to SWD activity.
4. SWDIO line physically responds to SWDSelect sequence (voltage measured at SWD pin 2 drops from 3.17V to 3.09V during sequence).
5. However, ALL DP register read/write operations return ERROR:
- SWDReadDP 0 = ERROR (cannot read IDCODE 0x6BA02477)
- SWDWriteDP 2 0x02000000 = ERROR
- JLINK_CORESIGHT_WriteAPDPReg returns -1 on every attempt
6. nrfjprog --recover -f NRF53 fails with error -21 after only 127ms (genuine recovery should take 15-30 seconds), indicating the CTRL-AP is also inaccessible.
J-LINK LOG EXTRACT
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JLINK_CORESIGHT_Configure() - 100.365ms returns 0 [OK]
JLINK_CORESIGHT_WriteAPDPReg(DP reg 0x02, 0x00000000) - 0.501ms returns -1 [FAIL]
[All subsequent DP register writes return -1]
CONCLUSION
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The SWD physical layer is functional (VTref OK, SWDIO responsive, MCU actively running and reacting to SWD sequences), but the Debug Port does not respond to any SWD transactions. This behavior is consistent with APPROTECT being enabled at the factory in a way that blocks even CTRL-AP access for recovery.
Both modules were purchased from amazon.com on May 26, 2026 .
QUESTIONS
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1. Are MDBT53V-1M modules shipped with APPROTECT enabled by default?
2. If so, is there a factory unlock procedure or is a special firmware pre-programmed?
3. Can you confirm whether modules from our lot have any factory firmware that could block SWD access?
Please find the full J-Link log file attached.
I would appreciate any guidance on how to proceed.
Thank you for your support.
