In a typical microcomputer reset circuit, one or more of the microcomputer's outputs are monitored to determine if a hardware reset should be invoked, and if so, to generate repetitive resets until normal microcomputer output is detected. A typical reset circuit (FIG. 1) employs a comparator circuit with hysteresis connected in a feedback configuration to provide an oscillating high/low output from which the repetitive reset signal is generated.
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MOTOROLA Technical Developments Volume 11 Cktober 1990
SIMPLIFIED MICROCOMPUTER RESET CIRCUIT
by Eugene L. Wineinger
In a typical microcomputer reset circuit, one or more of the microcomputer's outputs are monitored to determine if a hardware reset should be invoked, and if so, to generate repetitive resets until normal microcomputer output is detected. A typical reset circuit (FIG. 1) employs a comparator circuit with hysteresis connected in a feedback configuration to provide an oscillating high/low output from which the repetitive reset signal is generated.
The simplified reset circuit (FIG. 2) obviates the need for a comparator circuit by utilizing onboard hardware of the microcomputer to provide the neceesary hysteresis. The inherent hysteresis of the microcomputer's RESET input is used in place of the hysteresis formerly provided by the comparator circuit, and an additional high/low port of the microcom- puter is used to replace the output of the comparator circuit. The only other components used are passive components which are also used in the FIG. 1 circuit.
The use of the additional microcomputer port as a high/low port is possible since the microprocessor, while in the reset state, places the port in a high impedance state. This high impedance state essentially prevents the port's output from affecting the external reset circuit, which is designed to then produce a high voltage at the node connected to the additional microcomputer port. The low sta...
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