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'Connect under reset' is an STM32 thing, nrf52840 can't really do that and the probe doesn't use hardware reset pin. Once the MCU is out of pin reset, Ctrl-AP can hold the mcu in soft reset, but that's the standard behavior.

The difference between Pico and Pi Debug Probe are those 100Ω resistors on CLK/DIO lines, so I guess it working for you means I need to try again with resistors soldered.

Thanks for verifying it works!

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There's basically zero chance 100Ω resistors are contributing to your issues on a protocol that is host-clocked and can operate in the single kilobaud range.

Lower speed or try one of the other fifty debug firmware projects.

https://github.com/raspberrypi/debugprobe

https://github.com/bugadani/rusty-probe-embassy

https://github.com/probe-rs/rusty-probe

https://github.com/ciniml/rust-dap

https://github.com/ccattuto/rp2040-dap-probe

Not all firmware implements all modes or the CMSIS-DAP spec correctly (most don't implement JTAG, either.)

Okay i'm officially a dumbass - looking at the pinout i can clearly see Arduino pins and normal GPIO pins are mixed up on the board I used (seeed xiao rp2040). Arduino strikes again!

Just checked - of course if works fine on proper pins

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