RM0438 RNG htcr reg config && rng_cr

[Wxu.1]

hi there

I cannot figure out the true meaning of htcr reg and config1~config3 field in rng_cr reg。 I was wondering about the relationship between health test register and health test parameter （repetition counter test the “40�? consecutive bits or adaptive proportion test the "691" times)

asking for help!!! any reply will be welcome ​

[TZARDI]

Hello,

The register HTCR is used for the health checks. It ensures that the entire entropy source starts then operates as expected, obtaining assurance that failures are caught quickly and with a high probability and reliability. And for its start up tests it includes a test that verifies that on one 1024 bit window the first bit is not repeated more than 691 times for example if we have a window of 1024 bits and its first bit is 1 then the number of 1s has to be inferior than 691. It also has a test that flags an error when a bit is repeated consecutively 40 times (example 1111... 40times an error occurs) ...

It is recommended to use RNG_HTCR= 0x0000A2B3 and make sure that this magic number 0x17590ABC is written immediately before that value:

1. RNG->HTCR = 0x17590ABC;
2. RNG->HTCR = 0x0000A2B3;

The config1~config3 in rng_cr register they controls the configuration of the entropy sources and it is recommended to use the tested values specified in the Table 214. RNG configurations of the RM0438.

I hope that my response is helpful.

Best regards,

[Wxu.1]

thank you for your kindly answer TZARDI ，I still have two question

1�?my confusion is HTCR reg as recommended is 0x0000A2B3 ，would you give some more info about the value， why A2B3 ？is it the parameter related to 691 times repetition or 40 times. and for config1~config3 the RM0438 only give the recommend vlaue but without any explainnation about how to using these configs .

2�?post_processing in this RM is described as half inverter half normal sample ， is there any other special operation for post_processing 。 how do I know if post_processing is included during application consumption.

appreciate for any info

​

[TZARDI]

Hello,

1.Yes, the value of the HTCR register is related to both the 691 times repetition and 40 times consecutive repetitions. The config1~config3 are related to the configuration of the internal oscillators(number of cycles before start up, number of oscillators working in parallel...)

The values of the HTCR and CR registers are the result of the characterization process so to ensure the right performance of the IP they have to be respected.

2.In Post processing a normalization debiasing is applied and it is not applied only in NIST configuration: when NISTC bit =0.

Br,

[Wxu.1]

so

1�?in configA for respect nist sp800 90B rng_config1 == 0xF means 16 cycles before startup（likely for oscillator the warm up time？chapter28.3.4 RM0438 Rev5 page941）and config2 config3？ would you mind giving more detail info about these conifgs ，like config1 0xF stand for 16 cycles before start up（maybe this is not right）.

2�?HTCR is related to 691 and 40 but I cannot figure how to map these to 0xA2B3​，would you detail it？

3�?table 210 still bother me with NB loop param，what is this for？ no more explanation in this doc.

4�?chapter 28.5 f_threshold = 1.6xf_AHB? f_rng = 48 f_rng*1.6=77?

thank you TZARDI

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[TZARDI]

Hello,

-NB loop is the number of loop required for conditioning stage.

-f_threshold is used to determine the time between two sets of four 32-bit data

with fthreshold = 1.6 x fAHB example when fAHB= f_rng = 48MHz -> fthreshold= 77 MHz

I hope my response is useful. If you need more details please contact your FAE to provide additional sensitive information. 

Best regards,