S32K312 - Bus fault when Reading Secure Debug Password

[Bijam]

Hello,

On the S32K312, I'm trying to activate the Secure Debug feature using a routine in my embeded software

My code is doing the following :

- Read Secure debug Password (CUST_DB_PSWD_A @ UTEST 0x1B00 0080)

- if the Password is not set (all bytes @ 0xFF), program it

What I'm observing is that when the device is in Lifecycle CUST_DEL, this code is working correctly.
However, when the lifecycle is advanced to either OEM_PROD or IN_FIELD, reading  the CUST_DB_PSWD_A triggers a bus fault exception.

I was not able to identify in the reference Manual that this was the expected behavior.
Is this normal ?
And if yes, how can the embedded code check if a custom secure Debug password is programmed or not, without trigerring an exception ?

Thank you

[Bijam]

Thanks @Valval ,

I'm assuming I can read the current LC state using the register "LC and LC Control (DCMLCC)".
There are two fields inside this register: "DCMRLC - Real LC" and "DCMCLC - Current LC".
What is the difference betwen both field ? I don't really understand what "Real LC" means.

[VaneB]

Hi @Bijam 

Your understanding is correct. The LC can be read from the DCMRLC and DCMCLC fields.

• DCMRLC (Real LC) reflects the actual LC value stored in the UTEST memory.
• DCMCLC (Current LC) represents a combination of the real LC and any simulated LC:

VaneB_0-1788192004223.pngVaneB_1-1788192011054.png

For example, if a simulated LC advancement is performed to the HSE_LC_SIMULATED_IN_FIELD stage, the DCMRLC (Real LC) should remain CUST_DEL, while the DCMCLC (Current LC) would report IN_FIELD. After a reset, the simulated state is cleared, and both fields should again indicate CUST_DEL.

If simulated LC advancement is not used, DCMRLC and DCMCLC should always report the same value.

[VaneB · Accepted solution]

Hi @Bijam 

Since the Lifecycle (LC) has already been advanced, there is no need to verify whether the password has been programmed.

Whether HSE firmware is used or not, secure debug access can only be enabled after the debug password has been configured and the device LC has been advanced. The LC cannot be advanced unless the password has already been programmed.

If you still need to confirm that the password is present, the best approach is to check the current LC state first. Only attempt to read/program CUST_DB_PSWD_A when the MCU is in CUST_DEL. If the MCU is already in OEM_PROD or IN_FIELD, skip the password verification entirely. The password must already be programmed.

BR, VaneB