I am trying to evaluate ways to protect my embedded code from being read and copy from outside world.
I wonder what are the popular ways used out there.
Pls note that you still need the option to upgrade the firmware later on.
This might not help if you are specifically using the TM4C129, however the ARMv8-M architecture adds features to support execute only memory. Which can be used to protect firmware from being read out of the device.
Whitepaper - ARMv8-M Architecture Technical Overview
Yes, I know this feature, however, when you do so, ALSO your code cannot read from FLASH.
You might need that from time to time, e.g: constants, tables...