Phillip Pearson - web + electronics notes

tech notes and web hackery from a new zealander who was vaguely useful on the web back in 2002 (see: python community server, the blogging ecosystem, the new zealand coffee review, the internet topic exchange).


CMSIS-DAP work: implemented raw JTAG support, and ported the HID firmware to Pro Micro and Teensy 3.2

While I am so happy to have my J-Link back, the couple of weeks without it have been very productive in terms of open source contributions. After finding out that OpenOCD didn't support raw JTAG mode on CMSIS-DAP adapters, I bit that off as a potential project, and eventually got it working, then did some performance tuning, and I'm pretty pleased with it now. With my LPC-Link2, it can program the flash in an ATMEGA32U4 over JTAG at about 1/4-1/3 of the speed of the J-Link (which is kind of a speed demon). I'm going to let it soak on GitHub for a while, then clean it up and submit it to OpenOCD once it's had a bit of scrutiny.

Clone myelin/openocd and check out the cmsis-dap-jtag branch to try it out.

Implementing this required getting pretty familiar with the CMSIS-DAP source code and protocol, and at some point I realized that it wouldn't be super hard to port the SWD/JTAG debugger part of the CMSIS-DAP firmware over to any USB capable microcontroller. Full CMSIS-DAP support requires the debugger part, plus a serial port and a mass storage device emulator that's also capable of flashing a .bin file to a chip, but I'm not doing this to provide a USB interface to a custom board (like the ones on the mbed site), so I've skipped those two. I'll probably add in the serial port sometime, because I wrote serial bridge code for teensy-openocd-remote-bitbang already.

This was mainly a matter of getting rid of Keil-specific code, plus a small amount of debugging:

  • The _OUT functions (e.g. PIN_SWDIO_OUT) take a uint32_t with a boolean value in the LSB, but often junk in the higher bits.
  • A 32-bit processor is expected, so there was one point where I needed to add (uint32_t) casts to avoid losing the high bytes of a word.

So here's CMSIS-DAP firmware for your Pro Micro (ATMEGA32U4) or Teensy 3.2 (MK20DX256) board.