The TI's TM4C series devices allow to disable the debug interface in order to prevent accesses to the device through the JTAG / SWD interface. Standard IEEE instructions (e.g. boundary scan operations) are still possible. Once the debug interface has been disabled, the only way to recover is to perform a special recovery sequence, described in the TM4C reference manual.
In order to disable the debug interface, the the DBG1bit of the Boot Configuration Register (BOOTCFG) needs to be changed from 1 (HIGH) to 0 (LOW). This register is part of the so called Non-Volatile Registers and therefore, a special sequence is needed to change it. Below, we provide two examples which disable the debug interface. Please note, that a power-on-reset is required after performing the sequence.
For further information regarding this topic, please refer to the TI TM4C reference manual.
Recover a locked device
A locked device can be recovered using the J-Link Commander. Select JTAG as target interface. Enter the unlock LM3Sxxx command and follow the instructions.
Script files and sample projects
J-Link Commander: This J-Link Command File changes the DBG1 bit in the BOOTCFG register from 1 to 0.
J-Flash: The disable debug interface sequence has been implemented as part of the exit steps which are executed on "Auto" programming (F7), only.