dutchanddutch/jbang

Quick hack showing how to bit-bang the JTAG port using pin configuration

jbang

This quick and dirty hack allows the BeagleBone Black (and more generally any other AM335x) to self-jtag without external hardware by manipulating the pinmux registers to make the processor think the pins are toggling.

The main purpose of this is to make ICEPick assert the DBGEN signal to the Cortex-A8, which allows the OS to use processor debug features such as watchpoints and more generally enable "debug monitor" mode. It would in principle also allow halting debug to be performed on the cortex-a8 with assistance of code running e.g. in PRUSS.

(Note: it turns out the hardware breakpoint/watchpoint code in linux does not currently support the ARMv7 memory-mapped debug model used by the Cortex-A8. Bummer.)

Hardware requirements

All that is required is that all relevant JTAG inputs (nTRST, TMS, TDI, TCK) are logic-high. The only obstacle on the BeagleBone Black is the pull-down resistor R25 on nTRST which needs to be removed or overridden e.g. by connecting nTRST (P2 pin 2) to VDD_3V3B (available on P2 pin 5 "TVDD").

By default the code also expects TDO (P2 pin 7) to be connected to EMU0 (P2 pin 13) to allow readback of JTAG data. EMU0 is just used as a conveniently nearby GPIO, you can use any other by changing src/hw-subarctic.cc. You can also disable TDO readback entirely by setting has_tdo to false in hw-subarctic.h, in which case the demo will just blindly perform its writes.

Software overview

The include/ dir is a bunch of common files from my baremetal projects, here reused to directly meddle with the hardware via /dev/mem. Most of these registers are also managed by the kernel, so please don't view this code as a good example. It's gross, but it was just the easiest way for me to verify this trick really works.

The jtag bit-banging demo itself (src/jbang.cc) is however reasonably stand-alone and should be easily ported to any other mechanism to control the JTAG port.

Oh, and yeah the whole thing is written in my rather eccentric style of C++. It requires gcc 4.9 to compile, older versions will not work. It should be readable enough if you pretend it's some unfamiliar C++-ish language, but if you're really trying to make sense of it, you may wish to start by reading include/defs.h.