nRF52840 custom PCB firmware does not execute after power-cycle unless J-Link/RTT is attached

[Ahmed0]

Hardware:

• Custom PCB, nRF52840 SoC
• VDD = 1.8V (battery-powered, no USB)
• External LFCLK crystal (32.768 kHz) originally configured; switched to RC oscillator during debugging

SDK / Toolchain:

• nRF Connect SDK v3.2.1 (Zephyr 4.2.99)
• Zephyr SDK 0.17.0
• nrfjprog + JLinkARM DLL for flashing/debug
  
  

What we have ruled out or tried:

1. RTT/logging backend – Fully disabled (CONFIG_LOG=n, CONFIG_USE_SEGGER_RTT=n, CONFIG_LOG_BACKEND_RTT=n, CONFIG_PRINTK=n, CONFIG_EARLY_CONSOLE=n, CONFIG_BOOT_BANNER=n, CONFIG_NCS_BOOT_BANNER=n). Issue persists.

2. LFCLK source – Was CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=y. Switching to CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y reduced idle current by ~100 µA but did not resolve the standalone boot issue.

3. Early return 0; exits in main() – Audited the entire main() function; it has several points where it returns early on init failure (GPIO not ready, IRQ config failure etc.). We added retry logic with a 5-second timeout and removed all early return paths to make startup tolerant of peripheral init failures. Issue persists.

4. APPROTECT – CONFIG_NRF_APPROTECT_LOCK=n confirmed; access port protection is not engaged.

Question to Nordic DevZone:

1. On nRF52840 at VDD = 1.8V, is there any condition under which the chip will remain in a low-power (~10–100 µA) non-executing state after power-on that would be resolved by J-Link probe attach? Specifically: is there any boot-mode selection, debug interface lock, or internal boot ROM behavior that could gate firmware execution on SWD/debugger presence?

2. Does the nRF52840 boot ROM perform any check or handshake when CONFIG_CLOCK_CONTROL_NRF_K32SRC_XTAL=y and the LFXO fails to stabilize? Could a slow-starting or absent XTAL cause the SoftDevice or clock driver to stall pre-main()?

3. Are there any NCS v3.2.1 / Zephyr 4.2.99 known issues or errata with cold-boot behavior at 1.8V that could cause the described "kickstart required" symptom? 

1. 
   

   CONFIG_GPIO=y
   CONFIG_PWM=y
   CONFIG_SPI=y
   CONFIG_ADC=y
   CONFIG_FLASH=y
   CONFIG_FLASH_MAP=y
   CONFIG_FLASH_PAGE_LAYOUT=y
   CONFIG_NORDIC_QSPI_NOR=y
   CONFIG_NVS=y
   CONFIG_MBEDTLS=y
   CONFIG_MBEDTLS_SHA256=y
   CONFIG_MBEDTLS_ENABLE_HEAP=y
   CONFIG_HEAP_MEM_POOL_SIZE=16384
   CONFIG_SENSOR=y
   CONFIG_LIS2DH=n
   
   # Clock: internal LFRC for custom PCB bring-up robustness
   CONFIG_CLOCK_CONTROL_NRF=y
   CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y
   
   # Logging: disable SEGGER RTT backend for production
   CONFIG_LOG=n
   CONFIG_LOG_BACKEND_RTT=n
   CONFIG_LOG_PRINTK=n
   CONFIG_USE_SEGGER_RTT=n
   
   # Disable UART console and backend
   CONFIG_UART_CONSOLE=n
   CONFIG_LOG_BACKEND_UART=n
   CONFIG_SERIAL=n
   CONFIG_CONSOLE=n
   CONFIG_PRINTK=n
   CONFIG_EARLY_CONSOLE=n
   
   # Debug basics
   CONFIG_DEBUG=n
   CONFIG_ASSERT=n
   CONFIG_MAIN_STACK_SIZE=2048
   CONFIG_NCS_BOOT_BANNER=n
   CONFIG_BOOT_BANNER=n
   
   # Reset cause diagnostics (no RTT needed — LED pattern at cold boot)
   CONFIG_HWINFO=y
   CONFIG_REBOOT=y
   
   # Safety: never lock APPROTECT from this build
   CONFIG_NRF_APPROTECT_LOCK=n
   
   # Power Management: enable device suspend/resume for peripherals like ADC
   CONFIG_PM_DEVICE=y
   
   # Bluetooth
   CONFIG_BT=y
   CONFIG_BT_PERIPHERAL=y
   CONFIG_BT_DEVICE_NAME="TamperSeal"
   CONFIG_BT_DEVICE_APPEARANCE=0
   CONFIG_BT_MAX_CONN=1
   CONFIG_BT_BAS=y
   
   # BLE data throughput: 64-byte event notifications require MTU >= 67
   CONFIG_BT_L2CAP_TX_MTU=247
   CONFIG_BT_BUF_ACL_TX_SIZE=251
   CONFIG_BT_ATT_TX_COUNT=6
   
   
   

   
   
   
   

[Ahmed0]

I solved my issue. I had my pcb configured to be in 1.8V mode but  the LDO was still enabled. 

Put this in your device overlay file to disable regulator LDO:

#include <zephyr/dt-bindings/regulator/nrf5x.h>


&reg1 {
	regulator-initial-mode = <NRF5X_REG_MODE_LDO>;
};

For anybody else stumbling onto this forum post, here is another forum post with similar issues but  their problem was different, it might help if my post hasnt: https://devzone.nordicsemi.com/f/nordic-q-a/64575/flashing-nrf52840-module#:~:text=nRFGo%20Studio%20is%20very%20old,I%20hope%20this%20helps.