NRF52840 Cryptocell register setting reading problem

[Barry]

I am trying to set the Lifecycle state of the cryptocell, but it seems like I am having an issue.

I do 

/** Enable crypto cell to configure it **/
NRF_CRYPTOCELL->ENABLE = CRYPTOCELL_ENABLE_ENABLE_Enabled << CRYPTOCELL_ENABLE_ENABLE_Pos;

/** Config cryptocell resgister so Device root key can only be set once per reset*/
NRF_CC_HOST_RGF->HOST_IOT_LCS = CC_HOST_RGF_HOST_IOT_LCS_LCS_Secure << CC_HOST_RGF_HOST_IOT_LCS_LCS_Pos;

/** Disable crypto cell until next use**/
NRF_CRYPTOCELL->ENABLE = CRYPTOCELL_ENABLE_ENABLE_Disabled << CRYPTOCELL_ENABLE_ENABLE_Pos;

But then later when I enable the cryptocell again and read the value of HOST_IOT_LCS,

LCS is 2 (like I expected) but LCS_IS_VALID is 0 which means invalid.

Any ideas?

I am using zephyr and I am doing this configuration post kernel with a priority after CONFIG_KERNEL_INIT_PRIORITY_DEFAULT. So the cryptocell is already being initialized by hw_cc310_init in file hw_cc310.c

[Barry]

Also the documentation doesn't really mention this (I don't think) but to be clear, if I want to write to or read from CC_HOST_RGF the cryptocell must be enabled correct. It seems that way but I only figured that out by trial and error.

[Einar Thorsrud]

Hi,

Yes, you are right. The documentation is unclear on this.

And about the original question, it seems you are right on that point tas well.  The LCS_IS_VALID is a read-only field, but if you write to it, this will make it 1 regardless of what you write. It is intended to show if LCS is set, which is the only field in HOST_IOT_LCS that should be written to.

So in short, any writing to any part of the HOST_IOT_LCS register sets the LCS_IS_VALID field to 1.