Recover nRF5340 / MDBT53 after wrong DCDC config on LDO-only board

[Hejdern]

Hi Nordic,

I have a custom board using a Raytac MDBT53 module with nRF5340. The board is designed for 3.3 V / LDO mode, but during bring-up I likely flashed firmware/configuration that enabled DCDC or otherwise set the regulator mode incorrectly.

After that, three boards became unreachable over SWD. Before flashing, SWD worked on the same connector/setup, and the same debugger still works reliably with unaffected boards. Holding reset during recovery attempts has not restored access.

Relevant details:

• MDBT53 / nRF5340 custom board
• Intended config: LDO mode, LFRC, no external LF crystal
• Suspected bad config: DCDC / regulator mismatch
• SWD connector works on unaffected boards
• Limited access to module pins, since the nRF5340 is inside the MDBT53 module

Questions:

1. Can wrong DCDC/regulator configuration prevent SWD access on nRF5340?
2. If SWD can be reached, will nrfjprog --recover / ERASEALL clear the bad UICR/regulator setting?
3. Is there a recommended connect-under-reset or recovery sequence for this failure mode?
4. If DCDC-related pins are not accessible on the MDBT53 module, is there any practical way to recover the boards?

For future builds we now force LDO, use LFRC, and verify that generated HEX files contain no UICR writes. I am mainly trying to determine whether the three affected boards can still be recovered without direct access to internal module pins.

Thanks!

[Kazi Afroza Sultana]

Hello,

1. Can wrong DCDC/regulator configuration prevent SWD access on nRF5340?

Wrong DCDC configuration causes device to behave abnormal for example sudden reset or unresponsive. 

If the device resets or hangs or crashes at startup due to regulator misconfiguration, there is a high chance not to be reachable over SWD. As after each reset APPROTECT requires firmware re-enable debugger access before the next one. UICR includes regulator configuration registers like UICR.VREGHVOUT.

2. If SWD can be reached, will nrfjprog --recover / ERASEALL clear the bad UICR/regulator setting?

Yes, nrfjprog --recover or eraseall command does eraseall operation via CTRL-AP which erases flash, SRAM and UICR (which includes regulator configuration registers like UICR.VREGHVOUT).

3. Is there a recommended connect-under-reset or recovery sequence for this failure mode?

You can read this blog (+) Allowing debugger access to nRF5340 - Blogs - Nordic Blog - Nordic DevZone

4. I need to talk to team to be sure of this query. 

Thanks.

BR
Kazi