Circuit detects reset source

Shyam Tiwari, Sensors Technology Pvt Ltd, Gwalior, India -- EDN, 4/27/2000

Embedded single-board computers do not rely only on power-on resets; they often use multiple sources to reset CPUs to prevent CPUs from being locked into endless loops. However, in most cases, CPUs start from the starting points of memories to fetch their first code, because they have no way of knowing what generated the reset. The circuit in Figure 1 allows a CPU to know the source of a reset. It stores in an 8-bit latch reset-control input data that the CPU can read. If an input frequently resets the CPU, the CPU can then report the error source to the user, using LEDs or other indicators. In the circuit of Figure 1, an experiment combined eight independent reset-signal sources into a wired-OR single output. The 74HCT574 stores the 8-bit reset data at the rising edge of the signal. The latch records no other signal if the signal appears after the rising edge of the first signal. If the CPU finds data that resembles a reset signal in the latch that the signal does not reset, then the reset pulse is too narrow to effect a reset. The CPU recognizes this signal after reading the information from the latch. The output of the latch is a tristate structure; the CPU reads the output using active chip-enable and read-input signals. (DI #2516)