nRF52840 startup delay

RE: nRF52840 startup delay

Romain — Mon, 22 Nov 2021 14:54:19 GMT

Hi Simon,

Indeed this is ~434ms faster when I set CONFIG_ENTROPY_CC3XX=n

Thank you.

RE: nRF52840 startup delay

Simon — Fri, 19 Nov 2021 14:51:00 GMT

It is the function nrf_cc3xx_platform_init() specifically that causes the long startup time.

You need to initialize the RNG engine in CryptoCell and it takes time.

The only option is to not use the nrf_cc3xx for the RNG driver and instead use nRF5 HW peripheral.

The nRF5 RNG is simpler than the CryptoCell RNG. There are some differences between these, as stated in the nRF5 SDK documentation --> RNG:

CryptoCell CC310 (default for devices with CC310)

nRF HW RNG peripheral:

CTR-DRBG mode using mbed TLS (default for devices without CC310)

Raw mode

CC310 is the preferred backend on devices that support it, as it meets the NIST 800-90B3 and AIS-31 (Class “P2 High”) standards. The nRF HW RNG backend is available on all nRF5 devices. Devices that do not include CC310 should normally use the nRF HW RNG with mbed TLS CTR-DRBG. The mbed TLS CTR-DRBG code is standardized by NIST (NIST SP 800-90A Revision 1 ).

RE: nRF52840 startup delay

Simon — Wed, 17 Nov 2021 12:03:44 GMT

I did some measurements with an nRF52840 DK using the external 32k without MCUboot, running the BLE peripheral UART sample with the SoftDevice Controller (forgot to enable SPI)

I saw that the function mpsl_lib_init took 250 ms, which is expected, since it's waiting for the 32 kHz clock to get started (see ticket 116026). I saw that the function hw_cc3xx_init_internal() used 434ms. I will ask internally about the hw_cc3xx function and get back to you.

I tried to measure at the same places as you, at the start of pre kernel 2 and at the start of start of main, but it only took 54ms. However, the execution time of pre kernel 1 took 685ms (~=250ms + 434ms).

RE: nRF52840 startup delay

Romain — Wed, 17 Nov 2021 07:07:12 GMT

Hi Simon,

Any news on that ?

Best regards,

Romain

RE: nRF52840 startup delay

Simon — Mon, 08 Nov 2021 23:32:24 GMT

I'll take a look at this tomorrow

Best regards,

Simon