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Thread: JTAG with (not of) Teensy

  1. #1

    JTAG with (not of) Teensy

    I have an old Olimex parallel-port Wiggler clone JTAG, but no easy-to-use parallel port any more. So I need a new JTAG dongle.

    Looking at the OpenOCD docs, they have a bit-bang driver that (apparently) is intended to control a simple JTAG device using about a dozen single-character commands. It seems like that would be really easy to implement on a Teensy, so the Teensy would control the 5 JTAG lines and report the state of TDO back to OpenOCD.

    My question is, has anybody implemented this yet?

    I just built a recent version of OpenOCD, and I'm trying to get oriented by getting it to talk to an FTDI-2232 JTAG that's embedded into a Stellaris Evalbot. (Unfortunately, it's not a stand-alone FTDI JTAG dongle; the chip is hardwired to the rest of the Evalbot.) It's going slowly, but I expect I'll be able to get OpenOCD running without too much more trouble. Then on to figuring out how the bitbang interface is supposed to work...

  2. #2 Check out the BusBlaster.. Not quite as cheap as a Teensy, but still really well priced. It's supposed to be able to JTAG just about any device with a JTAG port, thanks to it's onboard CPLD.

  3. #3
    True, there's several options (including cheap ones) but I have some Teensies! Anyway, it turns out that there's an OpenOCD driver for the BusPirate, which I suspect will happily talk to appropriate firmware on a Teensy, too. (I wouldn't be surprised if it's very similar to the BusBlaster driver, but haven't checked.) I think I'll be able to write a quick Teensy program that enumerates as a serial port (PJRC already provides that code) and then just interacts with OpenOCD using the bit-banging interface. Will send up my implementation if I can get it working.


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