How to enable/disable UICR custom registers writes through APPROTECT?

[cpeng]

Hello Devzone:

Would I be correct if I assume enabling APPROTECT should prevent writes into UICR? - for other readers, NO, IT CANNOT PROTECT AGAINST SOFTWARE READS/WRITES, ONLY DEBUGGER

Based on this assumption I wrote the following code to enable APPROTECT:

NRF_NVMC->CONFIG = 0x1;//NVMC_CONFIG_WEN_Wen << NVMC_CONFIG_WEN_Pos; 
while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
NRF_UICR->APPROTECT = 0x00;
NRF_NVMC->CONFIG = 0x0; //NVMC_CONFIG_WEN_Ren << NVMC_CONFIG_WEN_Pos; 
while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}

And I indeed observed my writes into UICR are invalid because I can't read back the same values I wrote, read values are only 0xFFFFFFFFs.

The problem is now I can't re-enable UICR writes.

I thought replacing 2nd line in the above code with

NRF_UICR->APPROTECT = 0xFF;

would re-enable UICR writes, but in experiment my UICR writes are still invalid.

How do i disable APPROTECT and re-enable UICR writes?

Thanks!

Best regards.

[Turbo J]

UICR works like a flash page - you can erase it all at once only. Changing bits 1->0 is called "writing" but 0->1 can only be done with "erase".