nRF52840 dongle sometimes goes to the DFU mode at power-on

[RadoK]

Hi,

I have two nRF52840 dongles. Both configured to use external power source (SB2 cut, SB1 soldered).

The first one goes always to my nRF program on power on (battery connected). The second one often goes to the DFU mode on power-on (red LED pulsating). I would like the system to go to the program directly all the time.

Note: I saw a a 2 years old thread in Q&A that is basically same as mine. I followed hints from there but same situation here, no pins are used that are connected to the reset pin or DFU mode.

I do not see major difference between the boards except the VDD rail. Is there a recommendation how fast/slow should VDD rise? For the nRF52840 chip or directly for the dongle as it has resistor and capacitor on the SW2 DFU button connected to the RESET pin. Maybe the timing of the VDD rise is important here.

The working board uses TC1015 3.0V regulator with a small bypass capacitor (should have fast rise time) on the LDO BP pin. The PicoOLED board uses RT9193 3.3V regulator with 22nF bypass capacitor (should have slower rise time).

If needed I can probe the VDD and RESET lines with oscilloscope.

Thanks,
Rado

[RadoK]

Just a quick update. I added 10uF capacitor to the VDD rail. As addition to the original 1uF. It helped but not 100% time. Still sometimes it powers on to the DFU mode.