AN3307 datasheet(15/37 Pages) STMICROELECTRONICS

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AN3307

Class B software package

Doc ID 18186 Rev 4

15/37

Figure 1.

Example of RAM memory configuration

4.1.3

Class B flow control

A specific software method is used to check if all parts of the test flow are successfully

passed. Unique labels (constant numbers) are defined for identifying all key points (blocks

with component tests) in the code flow in order to make sure no block is skipped and all of

them are performed as expected. The unique labels are processed in two complementary

counters to comply with class B variable criteria. The main principle is a symmetrical

four-step change of the counter pair content (adding or subtracting the unique label values)

each time any significant testing block is processed. Two of the steps check if the block is

correctly called from the main flow level (processed just before calling and just after return

from the called procedure). The next two steps check if the block is correctly completed

inside the called procedure (processed just after entry and just before return from the

procedure). An example is given in Figure 2, where a routine performing a component test is

called in the checked flow sequence and the four-step checking service is shown. This

method decreases the load on the CPU as all these points are always checked by counting

one member of the complementary counter pair only. As there is always the same number

of call/return and entry/exit points, the values stored in the counter pair must be always

complementary ones after any block is passed completely. Several execution flow check

points are evaluated and placed in the code flow where the integrity of the counter pair is

checked. If the counters are not complementary or they do not contain the expected values

at any of these checkpoints, the Fail Safe routine is called.

RAM Buffer location + pointer

Class B variables

redundant complementary values)

Class B variables

(normal values)

Class A variables

A specific pattern is stored here to

detect stack overflow / unterflow

Stack

Unused RAM

MS18590V1

AN3307 Datasheet(PDF) 15 Page - STMicroelectronics