Establish early communication with ITM

[vkottler]

ARM documentation: https://developer.arm.com/documentation/100166/0001/Instrumentation-Trace-Macrocell-Unit?lang=en

I'm pretty sure we can use SWO viewer for this? https://wiki.segger.com/J-Link_SWO_Viewer

[vkottler]

https://electronics.stackexchange.com/questions/152315/printf-style-debugging-via-swd

[vkottler]

CMSIS: https://github.com/ARM-software/CMSIS_5

[vkottler]

The following section describes the ITM registers whose implementation is specific to this processor. Other registers are described in the Armv7-M Architectural Reference Manual.

We need to get ahold of the architecture manual as well..

(note: created vkottler/ifgen#57 while working on this)

[vkottler]

So far we've added "fields" for one register: https://developer.arm.com/documentation/100166/0001/Instrumentation-Trace-Macrocell-Unit/ITM-Trace-Privilege-Register--ITM-TPR?lang=en

Architecture manual available as PDF: https://developer.arm.com/documentation/ddi0403/ee

[vkottler]

Next step is to populate the rest of the register bit-fields for all of the other ITM registers, then try actually using the ITM.

From there we should be able to bootstrap everything else (assuming we can get the SWO-based bi-directional comm working!)

[vkottler]

https://developer.arm.com/documentation/ddi0484/c/Debug/About-debug

Doesn't seem like the M0+ has the debug interface/ITM, should we consider trying the Segger-specific RTT stuff?

[vkottler]

https://github.com/SEGGERMicro/RTT

seems like the thing to check out

[vkottler]

Yep. Those examples are what I need to get working it seems.

[vkottler]

Big success on this branch: #9.

Getting the "hello, world!" console spam:

Using JLinkSWOViewerCLExe. I was also able to get the same working with the GUI version.

I think next we want to try the GDB port integration!

[vkottler]

Open question: is there a reading end of this ITM? That would be super legit, I don't think there is though.

edit: but how do people say you can do semi-hosting with this, then?

https://wiki.segger.com/Semihosting

[vkottler]

Even more recent success on this front: #11.

I'd like to do a bit more tweaking so we can get this to true "one button workflow" level (figure out why we need to hit enter a few times when entering GDB + ensure that the program also starts with another continue by default).

[vkottler]

Once this is complete, we can start trying to implement character handling + echo and stuff? Basically make an initial CLI.

[vkottler]

See this comment: #6 (comment).

edit: we did also get output working via the ITM