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SM CODE
CPU_SM_5
It also contributes to dramatically reduce potential common cause failures, because the
external watchdog is clocked and supplied independently of
Device
.
Error reporting
Depends on implementation
Fault detection time
Depends on implementation (watchdog timeout interval)
Addressed fault model
Permanent/transient
Dependency on
Device
configuration
None
Initialization
Depends on implementation
Periodicity
Continuous
Test for the diagnostic
To be defined at system level (outside the scope of
Compliant item
analysis).
Multiple-fault protection
CPU_SM_1: Control flow monitoring in
Application software
Recommendations and known limitations
In case of usage of windowed watchdog,
End user
must consider possible tolerance in
Application software
execution to avoid false error reports (affecting system availability).
It is worth noting that the use of an external watchdog could be needed anyway when
Device
is used to trigger final elements, in order to comply at system level with requirements from
IEC61508-2:2010 Table A.1/Table A.14.
Table 9.
CPU_SM_6
SM CODE
CPU_SM_6
Description
Independent watchdog
Ownership
ST
Detailed implementation
Using the IDWG watchdog linked to control flow monitoring method (refer to CPU_SM_1)
addresses failure mode of program counter or control structures of
CPU
.
Error reporting
Reset signal generation
Fault detection time
Depends on implementation (watchdog timeout interval)
Addressed fault model
Permanent
Dependency on
Device
configuration
None
Initialization
IWDG activation. It is recommended to use
hardware watchdog
in option byte settings (IWDG
is automatically enabled after reset).
Periodicity
Continuous
Test for the diagnostic
WDG_SM_1: Software test for watchdog at startup
Multiple-fault protection
CPU_SM_1: Control flow monitoring in
Application software
WDG_SM_0: Periodic read-back of configuration registers
Recommendations and known limitations
The IWDG intervention is able to achieve a potentially “incomplete” local safe state because
it can only guarantee that
CPU
is reset. No guarantee that
Application software
can be still
executed to generate combinations of output signals that might be needed by the external
system to achieve the final safe state. If this limitation turn out in a blocking point,
End user
must adopt CPU_SM_5.
Table 10.
CPU_SM_7
SM CODE
CPU_SM_7
Description
Memory protection unit (
MPU
).
Ownership
ST
Detailed implementation
The
CPU
memory protection unit is able to detect illegal access to protected memory areas,
according to criteria set by
End user
.
Error reporting
Exception raise (MemManage).
UM2305
Hardware and software diagnostics
UM2305
-
Rev 10
page 14/110