AN3307 datasheet(34/37 Pages) STMICROELECTRONICS

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Class B solution structure

AN3307

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Doc ID 18186 Rev 4

5.4.7

Partial RAM runtime self-test

The partial transparent RAM test is performed inside the timebase interrupt service routine.

The test covers the part of the RAM allocated to class B variables. One block of six bytes is

tested in each step of the test. To guarantee coupling fault coverage, the blocks of memory

are overlapped by two side bytes from one test step to the other. The order of testing the

bytes in the block is scrambled with respect to the physical order of addresses in memory.

For more details see Section 5.2.4: Full RAM March C-/X self-test.

Figure 20.

Partial RAM runtime self-test structure

In the first phase, the content of the block is stored in the storage buffer. Marching

destructive tests are performed over all the bytes of the tested RAM block in the next phase.

Then, in the final phase of this step, the original content is restored from the storage buffer.

The March X algorithm is faster because two middle marching steps are skipped. As a last

step of the test sequence, the storage buffer itself is tested by a marching test. The buffer is

tested together with the next two additional bytes to cover coupling faults in the buffer itself.

Then the whole test is reinitialized and it starts from the beginning. If any fault is detected,

the Fail Safe routine is called.

RAM_pointer = End

of Class B RAM?

Save content of RAM

block into the Buffer

Apply March test to the

RAM block

Restore content of RAM

block from the Buffer

RAM_pointer += block_size

Test On going

Apply March test

to the Buffer

FailSa fe () routine

Test OK

RAM_pointer = Start

of Class B RAM

Yes

Fail

AI18017

AN3307 Datasheet(PDF) 34 Page - STMicroelectronics