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I have one Amontec JTAGKey2 Generic USB JTAG cable interface. What I am looking for is some explanation of how to "explore" a device of which I don't know all exact details, but for which I have a BSDL file that fits almost. I cannot damage the device, which is why I'm trying to be extra-cautious.

Is this even possible or am I looking at the wrong technology for the task below?:

  • In general my first main interest is to poke around in memory and perhaps change a few bytes.
  • Next would be flashing a component connected to the JTAG chain.

Right now I am locked out of the board because the "environment" of the (vendor-custom) boot loader is invalid and the boot loader simply ends up segfaulting without letting me into the interactive mode.

The board for which I only have very little documentation of a newer hardware revision (unfortunately under NDA) than I have is from RMI and can be used as a standalone network card or hosted as a PCI-X (not PCI Express!) board. The JTAG connector is a standard MIPS one and according to the vendor complies to the voltage and pinouts documented for MIPS.

The vendor only documents and supports use of particular (and rather expensive) JTAG probes. Amontec's JTAGkey2 isn't one of them.

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Quick Answer: You can't do it.

Long Answer: Yyyyyoooouuuu ccccaaaannnn''''ttt dddddooooo iiiittttt.

But seriously, without having a schematic you won't be able to drive the JTAG interface without running some risk of damaging things. JTAG lets you essentially set "most" of the pins on a chip into some sort of GPIO pins, and then lets you read or write their state. If you don't know what they are connected to then you run the risk of setting a pin to an output while some other pin is also driving it. That would create some "bus contention" which could damage the parts.

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