I use RTT fine without a J-Link (I have an ST-Link). There’s a fantastic Rust package called defmt-rtt that uses macros to leave all the bulky text in the ELF file and just stream the printf arguments. Super fast logging, as verbose as you want with very little overhead.

> If we don’t have a debugger connected, the breakpoint instruction is going to throw and exception.

Another fun trick: route all semihosting calls through this function in RAM:

  BX LR
  BX LR

  BKPT 0xAB
  BX LR

Two important notes:

1) Semihosting calls are extremely slow. They halt the processor until the debug probe polls the target, notices that it's stopped at a semihosting breakpoint, handles that, and resumes it. This can take tens to hundreds of milliseconds per call.

2) You technically need to initialize the semihosting file descriptors with an open() call before using them. Many debuggers will allow you to use the descriptors without initialization, but not all will.

This is super cool, but the best part of the article to me is that I now have a new favorite hexapeak word: 0xF0CACC1A! (Spoken as someone who has recently gotten into bread making).

This is really cool to see posted! Today I learned. I’ve used uart and RTT both but this is taking things a step further. Always love to see useful embedded content like this online and on hacker news

I always find Semihosting slow for logging purpose.