The Cortex-R4 architecture is supposed to set the CPU operation mode to ARM instruction mode when an interrupt occurs after coming from a reset.
Therefore, it's not possible to make a program using only the Thumb instruction set – even if you want to. In other words, you must make a program using both the ARM instruction set and the Thumb instruction set.
With that, make certain to follow these two points:
- Implement the exception vectors using ARM instructions.
- Implement the FIQ/IRQ interrupt handler using ARM instructions.
Depending on your tool environment, they could be implemented with C source code declaring a dedicated specifier for an interrupt function.
As reference, please refer to our sample program "RZ/T1 Group Initial Settings (r01an2554)" on our web site.