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**[tickets:#465] CMSIS-DAP SWD: "SWD ack not OK @ 0 JUNK" and "Error connecting
DP: cannot read IDR" **
**Status:** new
**Milestone:** 0.12.0
**Created:** Thu Feb 12, 2026 03:48 PM UTC by Shreyas Acharya
**Last Updated:** Thu Feb 12, 2026 03:48 PM UTC
**Owner:** nobody
**Attachments:**
- [Screenshot 2026-02-12
211553.png](https://sourceforge.net/p/openocd/tickets/465/attachment/Screenshot%202026-02-12%20211553.png)
(47.4 kB; image/png)
Hi,
I’m trying to program/debug an STM32F446RE (Nucleo-F446RE target MCU) using a
CMSIS-DAPv2 probe (DAP103 Bluepill CMSIS-DAP).
OpenOCD fails to connect over SWD.
Symptoms:
- Repeated: "SWD ack not OK @ 0 JUNK"
- Final: "Error connecting DP: cannot read IDR"
What works:
- Flashing the same firmware via onboard ST-LINK works (program + verify OK).
What fails:
- Using CMSIS-DAP from OpenOCD/PlatformIO. Earlier I also saw OpenOCD start
without interface cfg and error:
"Can't find openocd.cfg" and "Debug Adapter has to be specified" until I
forced the command line.
OpenOCD version:
- xPack OpenOCD 0.12.0-01004-g9ea7f3d64-dirty (2023-01-30)
Command used (direct OpenOCD):
openocd -d4 -f interface/cmsis-dap.cfg -f target/stm32f4x.cfg -c "transport
select swd; adapter speed 100; init; reset halt; exit"
Platform/OS:
- Windows (MSYS2/MINGW64 terminal)
Wiring:
- CMSIS-DAP GND -> target GND
- SWDIO -> SWDIO
- SWCLK -> SWCLK
- NRST -> NRST (optional / please advise)
- VTref/VTarget -> 3.3V (if required by CMSIS-DAP)
Notes/questions:
1) Could ST-LINK be contending on SWD lines on Nucleo-F446RE when using an
external probe? If yes, what’s the recommended way to disconnect ST-LINK from
the target?
2) Any recommended OpenOCD settings for CMSIS-DAP
(connect_under_reset/reset_config, lower speed, etc.)?
Full log (debug_level 4) attached below:
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