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Recovering a locked TM4C

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Peter Wittich edited this page Dec 16, 2019 · 18 revisions

The TM4C Tina microcontroller on the CM can lock up under certain circumstances, in such a way that the JTAG interface is no longer accessible for anything except the most basic boundary scan. You cannot use this to reload the board, for instance, nor can you debug the application. It’s not exactly clear to me how one gets into this situation, but it has happened numerous times.

One can recover the board with the SEGGER JTAG programmer with the following steps. This assumes you are using a mated SM+CM.

  1. Connect the segger to the front panel JTAG connector.
  2. Ensure that the SM does not automatically power on the CM. (as of 12/2019 this requires manipulation of the Zynq software.) This means that the MCU is held in reset since power-on.
  3. Open the JLinkExe program.
  4. Type unlock LMS3xxx 1. Don’t yet power cycle.
  5. Repeat previous step.
  6. From BUTool.exe, write 1 to the MCU to take it out of reset (CM.CM1.CTRL.ENABLE_UC)
  7. From BUTool.exe, write 0 to the MCU to return the MCU to reset (CM.CM1.CTRL.ENABLE_UC)
  8. Turn off the power on the Apollo

At this point the board should be recovered. You will have to reprogram the flash.

What about the JTAG jumpers?

When the MCU is really hosed the FPGAs have no power, and as such they won’t even respond to the JTAG boundary scan instruction. At this point I think you need to set the jumpers to take the FPGAs out of the chain.

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