%The certification process of safety critical products for European and 
%other international standards often demand environmental stress, 
%endurance and Electro Magnetic Compatibility (EMC) testing. Theoretical, or 'static testing',
%is often also required. 
%


%% INTRO
% the problem
% the solution
% why you would want to read the paper


The certification process of safety critical products for European and 
other international standards often demand environmental stress, 
endurance and Electro Magnetic Compatibility (EMC) testing. Theoretical, or 'static testing',
is often also required. Failure Mode effects Analysis (FMEA) is a tool used
for static testing. Its use is traditionally applied to hardware (electrical and mechanical) systems.
With the increasing use of micro-controllers in smart instruments and control
systems generally, software is increasingly being seen as a missing factor in FMEA analysis.
This paper takes a simple example of a hardware/software hybrid (an industry standard {\ft} input), analyses it
using hardware and software FMEA, and then discusses the effectiveness of  the
failure modelling from the perspective of the hybrid hardware/software sub-system.
This paper demonstrates the pitfalls and benefits of applying HFMEA and SFMEA
to a hybrid system.
%


%% MIDDLE
% some background
% how important software is today
%
Failure Mode Effects Analysis (FMEA), is a bottom-up technique that aims to assess the effect all
component failure modes on a system.
It is used both as a design tool (to determine weaknesses), and is a requirement of certification of safety critical products.
FMEA has been successfully applied to mechanical, electrical and hybrid electro-mechanical systems.

Work on software FMEA (SFMEA) is beginning, but
at present no technique for SFMEA that
integrates hardware and software models %known to the authors 
exists. FMEA performed on mechanical and electronic 
systems can be termed Hardware FMEA (HFMEA).
%
Software generally, sits on top of most modern safety critical control systems
and defines its most important system wide behaviour and communications.
Currently standards  that demand FMEA for hardware (e.g. EN298, EN61508),
do not specify it for Software, but instead specify, good practise,
review processes and language feature constraints.
% 
Where FMEA % scientifically 
traces component {\fms}
to resultant system failures, software has been left in a non-analytical
limbo of best practises and constraints.
Where software FMEA (SFMEA) has been applied, it is
performed a separately from the HFMEA.

%% CONCLUSIONS.
%
%
 
This paper presents an  analysis of a simple software/hardware hybrid sub-system (a {\ft} input circuit, MUX, ADC and two software functions 
that are used to convert the electrical current signal into a value for use in software).
HFMEA is applied to the hardware and SFMEA to the software components.
The two failure models are then compared, and then compared with heuristic
knowledge about {\ft} inputs circuitry and software.
  
Conclusions are then reached giving a positive and negative aspects
of analysing the hardware/software hybrid system using HFMEA and SFMEA.



