Power Optimization when using the Crypto Cell on nRF5340

[neo_here]

Hey, I have been trying to get to the root of additional current draw when crypto drivers are enabled. 

Without the crypto drivers enabled the current draw between two advertisements is around 3-5uA, with crypto drivers enabled it shoots up to 80uA. 

I am using the Crypto APIs for HMAC signing. 

These are the config options enabled in prj.conf 

CONFIG_TFM_PROFILE_TYPE_NOT_SET=y        

CONFIG_PSA_CRYPTO_DRIVER_OBERON=n

CONFIG_PSA_CRYPTO_DRIVER_CC3XX=y

CONFIG_NRF_SECURITY=y

CONFIG_MBEDTLS_PSA_CRYPTO_C=y

CONFIG_MBEDTLS_ENABLE_HEAP=n

As for the HMAC signing, I'm importing a key using psa_import_key() and signing using the psa_mac_ series of APIs for signing. 

I thought MBEDTLS might not be needed when hardware acceleration is used, but looks like it's being used during key import. 

 

[neo_here]

This is what it looks like on the PPK II

[dejans]

Hi,

Which board do you use?

Could you show full PPK2 measurement window?

Best regards,
Dejan

[neo_here]

Hey, we are working with the nRF 5340DK while custom boards are WIP. 

Here's the measurement window. 

Without crypto this is in the 4-7uA range. 

[neo_here]

I was wondering if there was any cryptocell/mbedtls specific power optimization needs to be done. 

Just including the below in the project increases this current draw without calling any crypto APIs

CONFIG_TFM_PROFILE_TYPE_NOT_SET=y        

CONFIG_PSA_CRYPTO_DRIVER_OBERON=n

CONFIG_PSA_CRYPTO_DRIVER_CC3XX=y

CONFIG_NRF_SECURITY=y

CONFIG_MBEDTLS_PSA_CRYPTO_C=y

CONFIG_MBEDTLS_ENABLE_HEAP=n

[neo_here]

I was able to get rid of 

CONFIG_NRF_SECURITY=y

CONFIG_MBEDTLS_PSA_CRYPTO_C=y

CONFIG_MBEDTLS_ENABLE_HEAP=n

from prj.conf and everything still works. Current consumption still the same