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APX803-44SA Datasheet(PDF) 7 Page - Diodes Incorporated |
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APX803-44SA Datasheet(HTML) 7 Page - Diodes Incorporated |
7 / 11 page APX803/D 3-PIN MICROPROCESSOR RESET CIRCUIT APX803 Document number: DS32132 Rev. 2 - 2 7 of 11 www.diodes.com September 2010 © Diodes Incorporated Timing Diagram Vth Vth /RESET RESET TDelay TDelay TDelay TDelay Vcc Vth Functional Description Microprocessors (µPs) and microcontrollers (µC) have a reset input to ensure that it starts up in a known state. The APX803/D drive the µP’s reset input to prevent code-execution errors during power-up, power-down, or brownout conditions. They assert a reset signal whenever the VCC supply voltage declines below a preset threshold and keep it asserted for a fixed period of time after VCC has risen above the reset threshold. For the APX803D this period is a minimum of 1ms while for other APX803 variants it is at least 140ms. The APX803/D have an open-drain output stage. Ensuring a Valid Reset Output Down to VCC = 0 RESET is guaranteed to be a logic low for VCC > 1V. Once VCC exceeds the reset threshold, an internal timer keeps RESET low for the reset timeout period; after this interval, RESET goes high. If a brownout condition occurs (VCC dips below the RESET reset threshold), RESET goes low. Any time VCC goes below the reset threshold, the internal timer resets to zero, and RESET goes low. The internal timer starts after VCC returns above the reset threshold, and RESET remains low for the reset timeout period. When VCC falls below 1V, the APX803/D RESET output no longer sinks current — it becomes an open circuit. Therefore, high-impedance CMOS logic inputs connected to RESET can drift to undetermined voltages. This presents no problem in most applications since most µP and other circuitry is inoperative with VCC below 1V. Interfacing to µP with Bidirectional Reset Pins Since the RESET output on the APX803/D is open drain, this device interfaces easily with μP/µC that have bidirectional reset pins, such as the Motorola 68HC11. Connecting the μP supervisor’s RESET output directly to the microcontroller’s ( μC’s) RESET pin with a single pull- up resistor allows either device to assert reset. Supervising and monitoring Multiple Supplies Generally, the pull-up resistor connected to the APX803/D will connect to the supply voltage that is being monitored at the IC’s VCC pin. However, some systems may use the APX803/D open-drain output to level-shift from the monitored supply to reset the µP powered by a different supply voltage or monitor multiple supplies that will be fed into 1 µC/µP reset input. |
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