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STM32F7 - Can not write to FPB registers

Question asked by bezak.branislav on Sep 25, 2015
Latest reply on Sep 29, 2015 by bezak.branislav
Hi All,
During conversion of our debugging code from Cortex-M3/M4 to M7 I run into following problem - it seems that it is not possible to write to any registers in FPB unit, for example FP_CTRL (0xE0002000) or FP_COMP0 (0xE0002008). When attempting to write the value, the register value does not change. Reading from these registers seems to work fine as I see expected value if I use external hardware debugger that sets these registers when breakpoint is set.

I have tried following:
- all this works fine on all STMF3 and STMF4 micros I have worked on
- verified that I'm using word access to these registers by checking generated assembly code
- verified that my code runs in privileged mode by checking value of Control register, I also tried to execute this from interrupt
- verified reading from these addresses if external debugger is setting them. Reading work OK
- tried writing various values, e.g. setting and clearing enable bit in FP_CTRL, verified that I'm writing proper key
- tried writing several FP_COMPx registers
- writing other registers in debugging block works fine, e.g. setting of CoreDebug->DEMCR works OK and I'm able to microstep the code
- checked Cortex-M7 documentation several times, it says that FPB should work the same as on Cortex-M3/M4 (ARMv7-M Architecture Reference Manual)

No matter what I do the value in registers does not change.

If anyone have any suggestions what to try next or what might be the problem I would appreciate it.