sw model notes

sw_model/sw_model.tex

@@ -290,6 +290,14 @@ have at least the following functional modiules intgrated into it:
 \item digital I/O. 
 \end{itemize}
 
+Need to consider the failure modes of all these components.
+
+For ADC we also need to consider the MUX and how it interfaces with general I/O 
+pins.
+
+this later makes the example where we test the ADC and the MUX by
+forcing logical values onto the I/O pins the ADC reads.
+
 
 %% Super bbc micro all on one 28 pin chip