Electronics > Microcontrollers

The meaning of DAP w.r.t M3 arm processor


[attachimg=1]Hi Everybody,

I need some assistance or basically an explanation. I am working on a project where some LPC1787 MCU are programming, and others not. It is probably a manufacturing issue. However I need to get to the bottom the issue.

My question:

In the IDE/debugger I use read the the following info from a debug log:

Thu Jun 08, 2023 10:05:22: Firmware: J-Link Ultra V4 compiled Sep 22 2022 15:00:10
Thu Jun 08, 2023 10:05:22: Selecting SWD as current target interface.
Thu Jun 08, 2023 10:05:22: JTAG speed is fixed to: 8000 kHz
Thu Jun 08, 2023 10:05:22: Found SW-DP with ID 0x2BA01477
Thu Jun 08, 2023 10:05:23: Failed to power up DAP
Thu Jun 08, 2023 10:05:24: Found SW-DP with ID 0x2BA01477
Thu Jun 08, 2023 10:05:24: Failed to power up DAP

What does DAP refer to and where can I read up more on its meaning ?

Any help or guidance on where to find some answers would be appreciated greatly!!
While googling I find people with similar issues, however no clear explanation regarding the meaning of "DAP" and "SW-DAP"

Thanks in advance :)

DAP - "Debug Access Port".

That's the part  of the ARM core that actually allows  you to debug the chip and also to program it.

That error means that your chip is talking to the debugger over SWD but the DAP is not responding - e.g. could be disabled by some configuration/fuse to prevent reverse engineering. You may have to clear that first somehow, if it is possible on your chip.

Or your programmer is not capable of programming that chip.



The other possibility is that your MCU is in JTAG and not SWD mode - this apparently depends on whether the /RESET is pulled low or high during the debug probe connection, according to the datasheet.

Or there is already software inside that change the gpio modes of the SWD pins.


Thank you both, Jeroen, and Janoc. I can not believe I did not know what DAP stands for.  :'(
However you have put me on the right track, and I am 85% sure the combination of your suggestions led me to finding the issue. Thanks so much!

As soon as I confirmed the issue properly I wil give feedback on what caused my issue.

Spoiler: Read your Errata


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