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Hercules™ Safety Microcontrollers Forum
NHET for Safety applications
I have some questions regarding the NHET modules of the TMS570.
My concern is the usage of NHET for safety applications. Since it is possible to change the NHET code during run-time by changing the NHET RAM directly in the main c-code. Which makes the NHET not really relevant for safety applications I guess?!?
1) When using NHET for safety applications are there any issues that have to be considered in order to prevent "violation of RAM access" from ""outside" e.g. Bit manipulation?
2) Are there any safety aspects of the NHET?
3) How can the NHET (RAM) be verified?
4) Is the NHET certified?
5 )What if I want to use for example NHET1 for QM part and NHET2 for ASIL part.How can I make sure that both NHETs won't interfere with each other?
6) Are there some guidelines how to write NHET programs so that such problems can not occur?
Thanks in advance!
The NHET (and previous HET) peripherals have been used in safety critical automotive applications for over a decade. NHET configuration in SRAM is inherently no different than other peripherals which have memory mapped control registers; if typical precautions with memory protection are taken, you should not see any problem in using the NHET.
1. Per bus master memory protection units can be used to block access to the NHET configuration memory on undesired accesses. You can also perform a periodic CRC on the NHET program memory using the CPU or DMA/CRC module to check NHET memory (assuming static programmation).
2. Please see the TMS570LS31x/21x safety manual available at www.ti.com/hercules for information on using the NHET in safety applications.
3. As noted in safety manual, NHET memory has built-in parity diagnostic, PBIST SRAM test capability, and is memory mapped to support additional testing via CPU.
4. I am not aware of any standard which would require review and certification of any peripheral for safety critical usage.
5. Each NHET has a memory protection unit upon its bus master. This can be used to limit access from each NHET to dedicated, non-overlapping memory spaces.
6. Please see the TMS570LS31x/21x safety manual available at www.ti.com/hercules for information on using the NHET in safety applications.
thanks for the answer!
Can you provide a simple code example of how you would execute a PBIST on NHET RAM?
In the Safety Manuel I read something about SRAM CRC-64 Testing which can be used to verify the contents of the NHET RAM
That sounds interesting but I couldn't find further information on that on how this can be implemented?
The HALCoGen tool should allow you to create a simple PBIST test for the NHET RAM.
For CRC testing, you can configure the DMA to read from NHET RAM and write to the CRC module to generate the CRC without CPU intervention. Optionally you can do the same with the CPU if you do not want to program the DMA.
In general, the safety manual will give you the hint of what you need to do to execute a test, but the TRM will provide the details of how to use a specific diagnostic. This is done to avoid duplication of content.
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