You can only access the original firmware if you can pass the following steps:
1. The chip operates.
2. The software should not be protected.
3. be protected should be set and meet the backdoor key.
4. If you have not configured the backdoor key you can not read the firmware in any way, unless you can enter it with a routine that give you a serial port and read the firmware and pull out the chip, and go capturing a PC. If well programmed this will not have access to run this routine.
If you have an OS you can access (if you know) the context of a task and change it to run yours, like a virus but is already more complex.