For some classes of applications, it may be desirable to know if the application of the reset signal to a microcontroller is due to an initial power-on sequence, or is the result of an external signal such as an operator pressing a reset pushbutton, or the result of a watchdog timer or similar event.
While there are perhaps numerous hardware solutions that can be employed, a simple
software solution can offer a high degree ofconfidence in making this determination. The task is to determine the differences in state of resources internal to the microcontroller that would occur as a result of these two types of reset conditions. With respect to the 80C51 family of microcontrollers, on-chip resources consist of the special function registers (SFRs) and the internal data memory (RAM). Most of the SFR locations are initialized as a result of a reset condition and thus cannot be used for this determination. The data memory contents are unaffected by reset. Thus, valid data loaded into the RAM of the 80C51 while
executing a program would not be affected by the application of an external reset signal provided the power source for the microcontroller has not been removed (as is the case for a “warm boot”).