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APX809-46SR Datasheet(PDF) 4 Page - Diodes Incorporated |
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APX809-46SR Datasheet(HTML) 4 Page - Diodes Incorporated |
4 / 9 page APX809/810 3-PIN MICROPROCESSOR RESET CIRCUITS APX809/810 Rev. 3 4 of 9 MARCH 2008 www.diodes.com © Diodes Incorporated Typical Application Circuit APX8XX GND RESET (RESET) V CC Microprocessor V CC GND RESET INPUT V CC CIN 100nF Functional Description A microprocessor’s (µP’s) reset input starts the µP in a known state. The APX809/810 assert reset 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, keeping it asserted for at least 240ms after VCC has risen above the reset threshold. The APX809/810 have a push-pull output stage. Ensuring a Valid Reset Output Down to VCC = 0 RESET is guaranteed to be a logic low for V CC > 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 APX809 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. However, in applications where RESET must be valid down to 0V, adding a pull down resistor to RESET causes any stray leakage currents to flow to ground, holding RESET low. R1’s value is not critical; 100k are large enough not to load RESET and small enough to pull RESET to ground. For the APX810 if RESET is required to remain valid for V CC < 1V. Benefits of Highly Accurate Reset Threshold Most µP supervisor ICs has reset threshold voltages between 5% and 10% below the value of nominal supply voltages. This ensures a reset will not occur within 5% of the nominal supply, but will occur when the supply is 10% below nominal. When using ICs rated at only the nominal supply ±5%, this leaves a zone of uncertainty where the supply is between 5% and 10% low, and where the reset may or may not be asserted. |
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