added read ADC function

papers/software_fmea/software_fmea.tex

@@ -387,10 +387,56 @@ int  read_4_20_input ( int * value ) {
 }
 }
 
+
+
+{\vbox{
+\footnotesize
+\begin{verbatim}
+/* 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;
+  /* require: input channel from ADC to be 
+              in valid ADC range         */
+
+  /* return out of range result  */
+  /* if invalid channel selected */
+  if ( channnel > 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 */
+  return dval; 
+}
+\end{verbatim}
+}
+}
+
+
 \subsection{FMMD Process}
 
 \paragraph{Functional Group - Convert mA to Voltage - CMATV}
 
+This functional group contains the load resistor 
+and the physical Analogue to Digital Converter (ADC).
+
 
 \paragraph{Functional Group - Software - voltage to per mil - VTPM }