Busfault when reading device unique id on stm32h7b3

[aco990]

Hello everyone,

i am trying to read the device unique id of my stm32h7b3 mcu, but get a bus fault on line 2:

 

 

 uint8_t* uid_base = (uint8_t*) 0x08FFF800;
 uint8_t i = uid_base[0];

 

 

Do you know what could be the problem ?

[aco990]

If i change the pointer to uint16_t* i can read without hardfault. But The reference manual says bytewise reading is possible. What do i wrong? On the stm32h743 i can read it bytewise.

@STOne-32 , @Peter BENSCH 
Best regards

[mƎALLEm]

Hello @aco990 ,

I reproduced the behavior on stm32h7b3 and not on STM32H743.

Question: did you enable the data cache? in that case are you facing the same behavior?

 

[aco990]

The issue is only on the stm32h7b3. The stm32h743 is working fine 

No the data cache is not enabled on both mcu

[mƎALLEm]

Hello,

Getting back with analysis of the behavior. 

In fact the behavior is expected with STM32H7B/A devices (not the case of STM32H74x/5x/2x/3x), and this is described in the RM0455 / "Single read sequence" section:

Since the UID is located at the address 0x08FFF800 accessed by the CPU over AHB, it could not be accessed with byte but with half-word or word.

From RM0455:

And in your example you're accessing the UID with bytes:

 uint8_t* uid_base = (uint8_t*) 0x08FFF800;
 uint8_t i = uid_base[0];