TM4C129: permanently locking JTAG

David M. Alter said:

But doesn't performing the unlock sequence described in section 4.3.4.3 restore the BOOTCFG register to the factory default settings, and therefore reverses the JTAG lockout (albeit the flash is also mass erased during the unlock procedure)?

Pretty much. And for good reason, people get annoyed when they brick the board.

David M. Alter said:

That seems like a difficult approach, since JTAG is generally needed during development, initial flash programming, etc.  Also, grounding the JTAG pins is not necessarily irreversible for a determined hacker.

There is no method of stopping someone who is determined, even secure micros (which the TM4C series most definitely is not) are breakable. It's all a question of

• providing evidence that you asserted your right (sort of a no trespassing sign). The question in that case is whether a simple label may be more cost effective
• increasing the cost to break. All these techniques cost the developer/manufacturer as well. So is this cost (repeated over N units) less than the cost to you of someone breaking the protection.

Robert 

Side note: I'd be surprised if there wasn't an inexpensive way to read the flash contents even with the JTAG disabled.

Hello David

Yes, unlocking sequence mass erases the device while unlocking the device, thereby "sort-of" securing the code. The tougher the access to JTAG pins, the more effort goes in for ??? value. Changing the JTAG pins to GPIO inputs also makes it a bit tougher even if the access to the pins is achieved.