TM4C123BH6PZ: can't download code to MCU

Hi, 

  Do you remember what was the program that was downloaded? A lot of time, the connection problem is caused by the running program that does one of the three things:

 - Put the device into hibernate or deepsleep mode without a means to wake up. When this happens, the debugger/programmer cannot synchronize with the target device anymore since the device is in sleep with all clocks turned off. 

 - Repurpose JTAG pins for GPIO mode. When this happens, the debug probe can longer connect to the device because there is no longer an operational JTAG interface. 

 - Configure the BOOTCFG register to disable JTAG interface for code protection. Normally, users choose this feature if they have a bootloader that can download code from a specified serial interface. But still no, debug capability is possible unless the chip goes through an unlock operation which is a mass-erase operation. 

 Was your code doing one of the above three?

 In LM flash programmer, try to restore the device to its factory setting by performing an unlock operation. There may be a chance of success but no guarantee. 

If all of the above do not solve your problem then it may be a hardware (either MCU or board level). You will need to perform a ABA swap test to find out what caused the problem. 

• A-B-A Swap Method
  (1) Remove the suspected component (A) from the original failing board.
  (2) Replace the suspected component (A) with a known good component (B) and check if the original board now works properly.
  (3) Mount the suspected component (A) to a known good board and see if the same failure occurs on the good board.