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5
papers/fmea_software_hardware/software_fmea.tex
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@ -734,7 +734,10 @@ hardware limitations etc.
The interface FMEA does serve to provide a useful
check-list to ensure data and synchronisation conventions used by the hardware
and software are not mismatched.
and software are not mismatched. However, the fact it is perceived as required
highlights the the miss-matches possible between the two types of analysis
which could run deeper than the mere interface level.
However, while these techniques ensure that the software and hardware is
viewed and analysed from several perspectives, it cannot be termed a homogeneous

2
submission_thesis/CH2_FMEA/Makefile
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@ -3,7 +3,7 @@
#
# Place all .dia files here as .png targets
#
DIA =
DIA = ftcontext.png
doc: $(DIA)

647
submission_thesis/CH2_FMEA/copy.tex
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@ -15,30 +15,36 @@ on the behaviour and safety of the system."
%\tableofcontents[currentsection]
FMEA is a broad term, and can mean anything from an informal check on how
how failures could affect some equipment in an initial brain-storming session
in product design to formal submissions as part of safety critical certification
procedures
This chapter describes the basic concepts, uses a simple example to
demonstrate an FMEA stage and then explores some concepts with which we can evaluate
the effectiveness of FMEA.
\subsection{FMEA}
This talk introduces Failure Mode Effects Analysis, and the different ways it is applied.
These techniques are discussed, and then
a refinement is proposed, which is essentially a modularisation of the FMEA process.
% \subsection{FMEA}
% This talk introduces Failure Mode Effects Analysis, and the different ways it is applied.
% These techniques are discussed, and then
% a refinement is proposed, which is essentially a modularisation of the FMEA process.
% %
%
\begin{itemize}
\item Failure
\item Mode
\item Effects
\item Analysis
\end{itemize}
% % \begin{itemize}
% \begin{itemize}
% \item Failure
% \item Mode
% \item Effects
% \item Analysis
% \end{itemize}
%
%
%
% % % \begin{itemize}
% % \item Failure
% % \item Mode
% % \item Effects
% % \item Analysis
% % \end{itemize}
\subsection{FMEA basic concept}
@ -53,6 +59,8 @@ a refinement is proposed, which is essentially a modularisation of the FMEA proc
FMEA is a procedure based on the low level components of a system, and an example
analysis will serve to demonstrate it in practise.
\subsection{ FMEA Example: Milli-volt reader}
Example: Let us consider a system, in this case a milli-volt reader, consisting
@ -81,10 +89,6 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry.
\subsection{FMEA Example: Milli-volt reader}
% \begin{figure}
% \centering
@ -95,7 +99,7 @@ For the sake of example let us choose resistor R1 in the OP-AMP gain circuitry.
\item \textbf{F - Failures of given component} The resistor (R1) could fail by going OPEN or SHORT (EN298 definition).
\item \textbf{M - Failure Mode} Consider the component failure mode SHORT
\item \textbf{E - Effects} This will drive the minus input LOW causing a HIGH OUTPUT/READING
\item \textbf{A - Analysis} The reading will be out of normal range, and we will have an erroneous milli-volt reading
\item \textbf{A - Analysis} The reading will be out of the normal range, and we will have an erroneous milli-volt reading
\end{itemize}
@ -112,20 +116,47 @@ Perhaps we should.... this would be a more rigorous and complete
approach in looking for system failures.
\subsection{Rigorous FMEA - State Explosion}
\subsection{Rigorous Single Failure FMEA}
Consider the analysis
where we look at all the failure modes in a system, and then
see how they can affect all other components within it.
\section{Theoretical Concepts in FMEA}
\subsection{The unacceptability of a single component failure causing a catastrophe}
FMEA, due to its inductive bottom-up approach, is very good
at finding potential component failures that could have catastrophic implications.
Used in the design phase of a project FMEA is an invaluable tool
for unearthing these type of failure scenario.
It is less useful for determining catastrophic events for multiple
simultaneous\footnote{Multiple simultaneous failures are taken to mean failure that occur within the same detection period.} failures.
\subsection{Impracticality of Field Data for modern systems}
Modern electronic components, are generally very reliable, and the systems built from them
are thus very reliable too. Reliable field data on failures will, therefore be sparse.
Should we wish to prove a continuous demand system for say ${10}^{-7}$ failures\footnote{${10}^{-7}$ failures per hour of operation is the
threshold for S.I.L. 3 reliability~\cite{en61508}.}
per hour of operation, even with 1000 correctly monitored units in the field
we could only expect one failure per ten thousand hours (a little over one a year) to fail.
It would be impractical to get statistically significant data for equipment
at these reliability levels.
However, we can use FMEA (more specifically the FMEDA variant, see section~\ref{sec:FMEDA}), working from known component failure rates, to obtain
statistical estimates of the equipment reliability.
\subsection{Rigorous Single Failure FMEA}
We need to look at a large number of failure scenarios
to do this completely (all failure modes against all components).
\subsection{Rigorous FMEA --- State Explosion}
FMEA cannot consider---for practical reasons---a rigorous approach.
It must be applied by experts in the system under investigation
to be a meaningful analysis.
\paragraph{Rigorous Single Failure FMEA}
FMEA for a safety critical certification~\cite{en298,en61508} will have to be applied
to all known failure modes of all components within a system.
To perform FMEA rigorously (i.e. to examine every possible interaction
of a failure mode with all other components in a system). Or in other words,
---we would need to look at all possible failure scenarios.
%to do this completely (all failure modes against all components).
This is represented in the equation below. %~\ref{eqn:fmea_state_exp},
where $N$ is the total number of components in the system, and
$f$ is the number of failure modes per component.
@ -138,23 +169,17 @@ $f$ is the number of failure modes per component.
\end{equation}
\subsection{Rigorous Single Failure FMEA}
This would mean an order of $N^2$ number of checks to perform
\paragraph{Rigorous Single Failure FMEA}
This would mean an order of $O(N^2)$ number of checks to perform
to undertake a `rigorous~FMEA'. Even small systems have typically
100 components, and they typically have 3 or more failure modes each.
$100*99*3=29,700$.
\subsection{Rigorous Double Failure FMEA}
For looking at potential double failure scenarios (two components
failing within a given time frame) and the order becomes
$N^3$.
\paragraph{Rigorous Double Failure FMEA}
For looking at potential double failure
scenarios\footnote{Certain double failure scenarios are already legal requirements---The European Gas burner standard (EN298:2003)---demands the checking of
double failure scenarios (for burner lock-out scenarios).}
(two components failing within a given time frame) and the order becomes $O(N^3)$.
\begin{equation}
\label{eqn:fmea_double}
@ -162,23 +187,24 @@ $N^3$.
%(N^2 - N).f
\end{equation}
$100*99*98*3=2,910,600$.
.\\
The European Gas burner standard (EN298:2003), demands the checking of
double failure scenarios (for burner lock-out scenarios).
For our theoretical 100 components with 3 failure modes each example, this is
$100*99*98*3=2,910,600$ failure mode scenarios.
\subsection{Four main Variants of FMEA}
\section{FMEA in practise: Five variants}
\paragraph{Five main Variants of FMEA}
\begin{itemize}
\item \textbf{PFMEA - Production} Car Manufacture etc
\item \textbf{FMECA - Criticallity} Military/Space
\item \textbf{FMEDA - Statistical safety} EN61508/IOC1508 Safety Integrity Levels
\item \textbf{DFMEA - Design or static/theoretical} EN298/EN230/UL1998
\item \textbf{SFMEA - Software FMEA --- only used in highly critical systems at present}
\end{itemize}
@ -417,6 +443,7 @@ programming languages and/or features.
\subsection{ FMEDA - Failure Modes Effects and Diagnostic Analysis}
\label{sec:FMEDA}
\textbf{Failure Mode Classifications in FMEDA.}
\begin{itemize}
\item \textbf{Safe or Dangerous} Failure modes are classified SAFE or DANGEROUS
@ -438,6 +465,7 @@ $ \sum \lambda_{SD}$, $\sum \lambda_{SU}$, $\sum \lambda_{DD}$, $\sum \lambda_{D
\subsection{ FMEDA - Failure Modes Effects and Diagnostic Analysis}
\textbf{Diagnostic Coverage.}
The diagnostic coverage is simply the ratio
of the dangerous detected probabilities
@ -558,3 +586,518 @@ judged to be in critical sections of the product.
\item Often Meeting notes or minutes only. Unusual for detailed arguments to be documented.
\end{itemize}
\section{Software FMEA (SFMEA)}
\paragraph{Current work on Software FMEA}
SFMEA usually does not seek to integrate
hardware and software models, but to perform
FMEA on the software in isolation~\cite{procsfmea}.
%
Work has been performed using databases
to track the relationships between variables
and system failure modes~\cite{procsfmeadb}, to %work has been performed to
introduce automation into the FMEA process~\cite{appswfmea} and to provide code analysis
automation~\cite{modelsfmea}. Although the SFMEA and hardware FMEAs are performed separately,
some schools of thought aim for Fault Tree Analysis (FTA)~\cite{nasafta,nucfta} (top down - deductive)
and FMEA (bottom-up inductive)
to be performed on the same system to provide insight into the
software hardware/interface~\cite{embedsfmea}.
%
Although this
would give a better picture of the failure mode behaviour, it
is by no means a rigorous approach to tracing errors that may occur in hardware
through to the top (and therefore ultimately controlling) layer of software.
\subsection{Current FMEA techniques are not suitable for software}
The main FMEA methodologies are all based on the concept of taking
base component {\fms}, and translating them into system level events/failures~\cite{sfmea,sfmeaa}.
%
In a complicated system, mapping a component failure mode to a system level failure
will mean a long reasoning distance; that is to say the actions of the
failed component will have to be traced through
several sub-systems, gauging its effects with and on other components.
%
With software at the higher levels of these sub-systems,
we have yet another layer of complication.
%
%In order to integrate software, %in a meaningful way
%we need to re-think the
%FMEA concept of simply mapping a base component failure to a system level event.
%
SFMEA regards, in place of hardware components, the variables used by the programs to be their equivalent~\cite{procsfmea}.
The failure modes of these variables, are that they could become erroneously over-written,
calculated incorrectly (due to a mistake by the programmer, or a fault in the micro-processor on which it is running), or
external influences such as
ionising radiation causing bits to be erroneously altered.
\paragraph{A more-complete Failure Mode Model}
% HFMEA
% SFMEA
% VARIABLE CURRUPTION
% MICRO PROCESSOR FAULTS
% INTERFACE ANALYSIS
%
% add them all together --- a load of bollocks, lots of impressive inches of reports that no one will be bothered to read....
%
In order to obtain a more complete failure mode model of
a hybrid electronic/software system we need to analyse
the hardware, the software, the hardware the software runs on (i.e. the software's medium),
and the software/hardware interface.
%
HFMEA is a well established technique and needs no further description in this paper.
\section{Example for analysis} % : How can we apply FMEA}
For the purpose of example, we chose a simple common safety critical industrial circuit
that is nearly always used in conjunction with a programmatic element.
A common method for delivering a quantitative value in analogue electronics is
to supply a current signal to represent the value to be sent~\cite{aoe}[p.934].
Usually, $4mA$ represents a zero or starting value and $20mA$ represents the full scale,
and this is referred to as {\ft} signalling.
%
{\ft} has an electrical advantage as well because the current in an electronic loop is constant~\cite{aoe}[p.20].
Thus resistance in the wires between the source and the receiving end is not an issue
that can alter the accuracy of the signal.
%
This circuit has many advantages for safety. If the signal becomes disconnected
it reads an out of range $0mA$ at the receiving end. This is outside the {\ft} range,
and is therefore easy to detect as an error rather than an incorrect value.
%
Should the driving electronics go wrong at the source end, it will usually
supply far too little or far too much current, making an error condition easy to detect.
%
At the receiving end, one needs a resistor to convert the
current signal into a voltage that we can read with an ADC.%
%we only require one simple component to convert the
%BLOCK DIAGRAM HERE WITH FT CIRCUIT LOOP
\begin{figure}[h]
\centering
\includegraphics[width=250pt]{./CH2_FMEA/ftcontext.png}
% ftcontext.png: 767x385 pixel, 72dpi, 27.06x13.58 cm, bb=0 0 767 385
\caption{Context Diagram for {\ft} loop}
\label{fig:ftcontext}
\end{figure}
The diagram in figure~\ref{fig:ftcontext} shows some equipment which is sending a {\ft}
signal to a micro-controller system.
The signal is locally driven over a load resistor, and then read into the micro-controller via
an ADC and its multiplexer.
With the voltage detected at the ADC the multiplexer we read the intended quantitative
value from the external equipment.
\subsection{Simple Software Example}
Consider a software function that reads a {\ft} input, and returns a value between 0 and 999 (i.e. per mil $\permil$)
representing the value intended by the current detected, with an additional error indication flag to indicate the validity
of the value returned.
%
Let us assume the {\ft} detection is via a \ohms{220} resistor, and that we read a voltage
from an ADC into the software.
Let us define any value outside the 4mA to 20mA range as an error condition.
%
As a voltage, we use ohms law~\cite{aoe} to determine the voltage ranges: $V=IR$, $$0.004A * \ohms{220} = 0.88V $$
and $$0.020A * \ohms{220} = 4.4V \;.$$
%
Our acceptable voltage range is therefore
%
$$(V \ge 0.88) \wedge (V \le 4.4) \; .$$
This voltage range forms our input requirement.
%
We can now examine a software function that performs a conversion from the voltage read to
a per~mil representation of the {\ft} input current.
%
For the purpose of example the `C' programming language~\cite{DBLP:books/ph/KernighanR88} is
used\footnote{ C coding examples use the Misra~\cite{misra} and SIL-3 recommended language constraints~\cite{en61508}.}.
We initially assume a function \textbf{read\_ADC} which returns a floating point %double precision
value representing the voltage read (see code sample in figure~\ref{fig:code_read_4_20_input}).
%%{\vbox{
\begin{figure}[h+]
\footnotesize
\begin{verbatim}
/***********************************************/
/* read_4_20_input() */
/***********************************************/
/* Software function to read 4mA to 20mA input */
/* returns a value from 0-999 proportional */
/* to the current input. */
/***********************************************/
int read_4_20_input ( int * value ) {
double input_volts;
int error_flag;
/* set ADC MUX with input to read from */
input_volts = read_ADC(INPUT_4_20_mA);
if ( input_volts < 0.88 || input_volts > 4.4 ) {
error_flag = 1; /* Error flag set to TRUE */
}
else {
*value = (input_volts - 0.88) * ( 4.4 - 0.88 ) * 999.0;
error_flag = 0; /* indicate current input in range */
}
/* ensure: value is proportional (0-999) to the
4 to 20mA input */
return error_flag;
}
\end{verbatim}
%}
%}
\caption{Software Function: \textbf{read\_4\_20\_input}}
\label{fig:code_read_4_20_input}
%\label{fig:420i}
\end{figure}
We now look at the function called by \textbf{read\_4\_20\_input}, \textbf{read\_ADC}, which returns a
voltage for a given ADC channel.
%
This function
deals directly with the hardware in the micro-controller on which we are running the software.
%
Its job is to select the correct channel (ADC multiplexer) and then to initiate a
conversion by setting an ADC 'go' bit (see code sample in figure~\ref{fig:code_read_ADC}).
%
It takes the raw ADC reading and converts it into a
floating point\footnote{the type `double' or `double precision' is a
standard C language floating point type~\cite{DBLP:books/ph/KernighanR88}.}
voltage value.
%{\vbox{
\begin{figure}[h+]
\footnotesize
\begin{verbatim}
/***********************************************/
/* read_ADC() */
/***********************************************/
/* Software function to read voltage from a */
/* specified ADC MUX channel */
/* Assume 10 ADC MUX channels 0..9 */
/* ADC_CHAN_RANGE = 9 */
/* Assume ADC is 12 bit and ADCRANGE = 4096 */
/* returns voltage read as double precision */
/***********************************************/
double read_ADC( int channel ) {
int timeout = 0;
/* return out of range result */
/* if invalid channel selected */
if ( channel > ADC_CHAN_RANGE )
return -2.0;
/* set the multiplexer to the desired channel */
ADCMUX = channel;
ADCGO = 1; /* initiate ADC conversion hardware */
/* wait for ADC conversion with timeout */
while ( ADCGO == 1 || timeout < 100 )
timeout++;
if ( timeout < 100 )
dval = (double) ADCOUT * 5.0 / ADCRANGE;
else
dval = -1.0; /* indicate invalid reading */
/* return voltage as a floating point value */
/* ensure: value is voltage input to within 0.1% */
return dval;
}
\end{verbatim}
\caption{Software Function: \textbf{read\_ADC}}
\label{fig:code_read_ADC}
\end{figure}
%}
%}
We now have a very simple software structure, a call tree, where {\em read\_4\_20\_input}
calls {\em read\_ADC}, which in turn interacts with the hardware/electronics.
%shown in figure~\ref{fig:ct1}.
%
% \begin{figure}[h]
% \centering
% \includegraphics[width=56pt]{./ct1.png}
% % ct1.png: 151x224 pixel, 72dpi, 5.33x7.90 cm, bb=0 0 151 224
% \caption{Call tree for software example}
% \label{fig:ct1}
% \end{figure}
%
This software is above the hardware in the conceptual call tree---from a programmatic perspective---%in software terms---the
software is reading values from the `lower~level' electronics.
%
%FMEA is always a bottom-up process and so we must begin with this hardware.
%
The hardware is simply a load resistor, connected across an ADC input
pin on the micro-controller and ground.
%
We can identify the resistor and the ADC module of the micro-controller as
the base components in this design.
%
We now apply FMMD starting with the hardware.
\section{Failure Mode effects Analysis}
Four emerging and current techniques are now used to
apply FMEA to the hardware, the software, the software medium and the software hardware insterface.
\subsection{Hardware FMEA}
The hardware FMEA requires that for each component we consider all failure modes
and the putative effect those failure modes would have on the system.
The electronic components in our {\ft} system are the load resistor,
the multiplexer and the analogue to digital converter.
{
\tiny
\begin{table}[h+]
\caption{Hardware FMEA {\ft}} % title of Table
\label{tbl:r420i}
\begin{tabular}{|| l | c | l ||} \hline
\textbf{Failure} & \textbf{failure} & \textbf{System} \\
\textbf{Scenario} & \textbf{effect} & \textbf{Failure} \\ \hline
\hline
$R$ & OPEN~\cite{en298}[Ann.A] & $LOW$ \\
& & $READING$ \\ \hline
$R$ & SHORT~\cite{en298}[Ann.A] & $HIGH$ \\
& & $READING$ \\ \hline
$MUX$ & read wrong & $VAL\_ERROR$ \\
& input ~\cite{fmd91}[3-102] & \\ \hline
$ADC$ & ADC output & $VAL\_ERROR$ \\
& erronous ~\cite{fmd91}[3-109] & \\ \hline
\hline
\end{tabular}
\end{table}
}
The last two failures both lead to the system failure of $VAL\_ERROR$ .
They could lead to low or high reading as well, but we would only be able to determine this
from knowledge of the software systems criteria for these.
\clearpage
\subsection{Software FMEA - variables in place of components}
For software FMEA, we take the variables used by the system,
and examine what could happen if they are corrupted in various ways~\cite{procsfmea, embedsfmea}.
From the function $read\_4\_20\_input()$ we have the variables $error\_flag$,
$input\_volts$ and $value$: from the function $read\_ADC()$, $timeout$, $ADCMUX$, $ADCGO$, $dval$.
We must now determine putative system failure modes for these variables becoming corrupted, this is performed in table~\ref{tbl:sfmea}.
{
\tiny
\begin{table}[h+]
\caption{SFMEA {\ft}} % title of Table
\label{tbl:sfmea}
\begin{tabular}{|| l | c | l ||} \hline
\textbf{Failure} & \textbf{failure} & \textbf{System} \\
\textbf{Scenario} & \textbf{effect} & \textbf{Failure} \\ \hline
\hline
$error\_flag$ & set FALSE & $VAL\_ERROR$ \\
& & \\ \hline
$error\_flag$ & set TRUE & invalid \\
& & error flag \\ \hline
$input\_volts$ & corrupted & $VAL\_ERROR$ \\
& & \\ \hline
$value $ & corrupted & $VAL\_ERROR$ \\
& & \\ \hline
$timeout $ & corrupted & $VAL\_ERROR$ \\
& & \\ \hline
$ADCMUX $ & corrupted & $VAL\_ERROR$ \\
& & \\ \hline
$ADCGO $ & corrupted & $VAL\_ERROR$ \\
& & \\ \hline
$dval $ & corrupted & $VAL\_ERROR$ \\
& & \\ \hline
\hline
\end{tabular}
\end{table} xe
}
\clearpage
\subsection{Software FMEA - failure modes of the medium ($\mu P$) of the software}
Microprocessors/Microcontrollers have sets of known failure modes, these include RAM, ROM
EEPROM failure\footnote{EEPROM failure is not applicable for this example.} and
oscillator clock timing
{
\tiny
\begin{table}[h+]
\caption{SFMEA {\ft}} % title of Table
\label{tbl:sfmeaup}
\begin{tabular}{|| l | c | l ||} \hline
\textbf{Failure} & \textbf{failure} & \textbf{System} \\
\textbf{Scenario} & \textbf{effect} & \textbf{Failure} \\ \hline
\hline
$RAM$ & variable & All errors \\
& corruption & from table~\ref{tbl:sfmea} \\ \hline
$RAM$ & proxegram flow & process \\
& & halts / crashes \\ \hline
$OSC$ & stopped & process \\
& & halts \\ \hline
$OSC$ & too & ADC \\
& fast & value errors \\ \hline
$OSC$ & too & ADC \\
& slow & value errors \\ \hline
$ROM$ & program & All errors \\
& corruption & from table~\ref{tbl:sfmea} \\ \hline
$ROM$ & constant & All errors \\
& /data corruption & from table~\ref{tbl:sfmea} \\ \hline
\hline
\end{tabular}
\end{table}
}
\clearpage
\subsection{Software FMEA - The software/hardware interface}
As FMEA is applied separately to software and hardware
the interface between them is an undefined factor.
Ozarin~\cite{sfmeainterface,procsfmea} recommends that an FMEA report be written
to focus on the software/hardware interface.
The software/hardware interface has
specific problems common to many systems and configurations
and these are described in~\cite{sfmeainterface}.
%An interface FMEA is performed in table~\ref{hwswinterface}.
%
The hardware to software interface for the {\ft} example is handled
by the 'C' function $read\_ADC()$
(see code sample in figure~\ref{fig:code_read_ADC}).
%
% An FMEA of the `software~medium' is given in table~\ref{tbl:sfmeaup}.
\paragraph{Timing and Synchronisation.}
The $ADCOUT$ register, where the raw ADC value is read
is an internal register used by the ADC and presented
as a readable memory location when the ADC
has finished updating it.
Reading it at the wrong time would
cause an invalid value to be read.
The synchronisation is performed by polling an $ADCGO$
bit, a flag mapped to memory by which the ADC indicates that the data is ready.
\paragraph{Interrupt Contention.}
Were an interrupt to also attempt to read from the ADC
the ADCMUX could be altered, causing the non-interrupt
routine to read from the wrong channel.
\paragraph{Data Formatting.}
The ADC may use a big-endian or little endian integer
format. It may also right or left justify the bits in its value.
\subsection{SFMEA Conclusion}
%
This paper has picked a very simple example (the industry standard {\ft}
input circuit and software) to demonstrate
SFMEA and HFMEA methodologies used to describe a failure mode model.
%Even a modest system would be far too large to analyse in conference paper
%and this
%
%The {\dc} representing the {\ft} reader
%shows that by taking a
%modular approach for FMEA, i.e. FMMD, we can integrate
Our model is described by four FMEA reports; and these % we can model the failure mode behaviour from
model the system from several failure mode perspectives.
%
With traditional FMEA methods the reasoning~distance is large, because
it stretches from the component failure mode to the top---or---system level failure.
%
With these four analysis reports
we do not have stages along the `reasoning~path' linking the failure modes from the
electronics to those in the software.
%Software is often written `defensively' but t
%Each {\fg} to {\dc} transition represents a
%reasoning stage.
%
%
%For this reason applying traditional FMEA to software stretches
%the reasoning distance even further.
%
In fact many these reasoning paths overlap---or even by-pass one another---
it is very difficult to gauge cause and effect.
For instance, hardware failures are not analysed in the context of how they will
be handled (or missed) by the software.
%
System outputs commanded from software may not take into account particular
hardware limitations etc.
The interface FMEA does serve to provide a useful
check-list to ensure data and synchronisation conventions used by the hardware
and software are not mismatched. However, the fact it is perceived as required %The fact its required
highlights the the miss-matches possible between the two types of analysis
which could run deeper than the mere interface level.
However, while these techniques ensure that the software and hardware is
viewed and analysed from several perspectives, it cannot be termed a homogeneous
failure mode model.
% For instance
% were the ADC to have a small value error, say adding
% a small percentage onto the value, we would be unable to
% detect this under the analysis conditions for this model, or
% be able to pinpoint it.
%
\section{Conclusion}
FMEA useful tool for basic safety --- provides statistics on safety where field data impractical ---
very good with single failure modes linked to top level events.
FMEA has become part of the safety critical and safety certification industries.
SFMEA is in its infancy, but there is a gap in current
certification for software, EN61508, recommends hardware redundancy architectures in conjunction
with FMEDA for hardware: for software it recommends language constraints and quality procedures
but no inductive fault finding technique.

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