The Circuit Designer's Companion - diagramas.diagram...

224 The Circuit Designer’s CompanionProgramROMUsedUnusedProgram & dataNOPNOPNOPNOPEmpty memoryNOPJMP RESETFigure 6.36 Protecting unused program memory with NOPs(safely) until it reaches the JMP RESET, at which point it restarts.The effectiveness of this technique depends on how much of the total possiblememory space is filled with NOPs, since the processor can be corrupted to a randomaddress. If the processor accesses an empty bus, its action will depend on the meaningof the #FF H instruction. The relative cheapness of large ROMs and EPROMs meansthat you could consider using these, and filling the entire memory map with ROM, evenif your program requirements are small.6.5.3 Re-initialisationAs well as RAM data, you must remember to guard against corruption of the set-upconditions of programmable devices such as I/O ports or UARTs. Many programmersseem to assume that once an internal device control register has been set up (usually inthe initialisation routine) it will stay that way forever. This is a dangerous assumption.Experience shows that control registers can change their contents, even though they arenot directly connected to an external bus, as a result of interference. This may haveconsequences that are not obvious to the processor: for instance if an output port is reprogrammedas an input, the processor will happily continue writing data to it obliviousof its ineffectiveness.The safest course is to periodically re-initialise all critical registers, perhaps in themain idling routine if one exists. Timers, of course, cannot be protected in this way. Theperiod between successive re-initialisations depends on how long the software cantolerate a corrupt register, versus the software overhead associated with the reinitialisation.