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debug-using-busblaster.md

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Debugging Tessel

You can debug Tessel by soldering JTAG headers to the board and using a JTAG debugger. This tutorial asummes you are using a Bus Blaster and an associated JTAG 20 -> 10 pin adapter.

Setup

  1. Install OpenOCD. You need version >= 0.8.0.

  1. Acquire a configuration script for your Bus Blaster. Included in the firmware repo is tools/tessel-busblaster.cfg.

  2. Plug the JTAG cable into the Bus Blaster's adapter board and the Tessel. Pin one is towards the USB port; the cable goes over the center of the board.

  3. Run

arm-none-eabi-gdb out/Release/tessel-firmware.elf -ex 'target remote | openocd -c "gdb_port pipe;" -f tools/tessel-busblaster.cfg'

(adjust paths as appropriate) to launch gdb.

GDB commands

Basics

  • c - continue
  • ctrl-c - stop
  • p expr - print the value of the C expression expr
  • bt - stack backtrace
  • s - step instruction, goes into calls
  • n - next instruction, skips over calls
  • fin - run until return from function
  • break function - break at the beginning of a function (can also pass file.c:line)
  • delete 1 - delete the first breakpoint
  • tbreak - break only once

Hardware

  • mon reset - Reset Tessel and all on-chip peripherals (like the reset button)
  • mon soft_reset_halt - Reset the CPU only

Other notes

  • Make sure your .elf file matches the firmware on the Tessel. Otherwise the position information will be wrong and calls will crash.

  • LPC1800 SPIFI Flash is functional from OpenOCD, but I don't recommend using it. Flash Tessel with the bootloader (make arm-deploy) before running GDB.

  • The ROM confuses JTAG on reset. If you c and then ctrl-c, it recovers. To debug the very beginning of execution, use

    mon soft_reset_halt
tbreak main
c

Troubleshooting

OS X: If you are getting a libusb error, you may need to disable some system drivers in order for OpenOCD to work. Run kextstat | grep -i ftdi. If any drivers are listed, you need to run either sudo kextunload -bundle com.apple.driver.AppleUSBFTDI and/or sudo kextunload -bundle com.FTDI.driver.FTDIUSBSerialDriver. For a long-term solution for configuring this, see http://alvarop.com/2014/01/using-busblaster-openocd-on-osx-mavericks/.