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MCP810M3X-2.93 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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MCP810M3X-2.93 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 7 page Applications Information Benefits of Precision Reset Thresholds A microprocessor supply supervisor must provide a reset output within a predictable range of the supply voltage. A common threshold range is between 5% and 10% below the nominal supply voltage. The 4.63V and 3.08V options of the MCP809/810 use highly accurate circuitry to ensure that the reset threshold occurs only within this range (for 5V and 3.3V supplies). The other voltage options have the same tight tolerance to ensure a reset signal for other narrow monitor ranges. See Table 1 for examples of how the standard reset thresholds apply to 3V, 3.3V, and 5V nominal supply volt- ages. TABLE 1. Reset Thresholds Related to Common Supply Voltages Reset Threshold 3.0V 3.3V 5.0V 4.63 ± 3% 90 - 95% 4.38 ± 3% 85 - 90% 4.00 ± 3% 78 - 82% 3.08 ± 3% 90 - 95% 2.93 ± 3% 86 - 90% 2.63 ± 3% 85 - 90% 77 - 81% Ensuring a Valid Reset Output Down to V CC =0V When V CC falls below 1V, the MCP809 RESET output no longer sinks current. A high-impedance CMOS logic input connected to RESET can therefore drift to undetermined voltages. To prevent this situation, a 100k Ω resistor should be connected from the RESET output to ground, as shown in Figure 1. A 100k Ω pull-up resistor to V CC is also recommended for the MCP810, if RESET is required to remain valid for V CC < 1V. Negative-Going V CC Transients The MCP809/810 are relatively immune to short negative-going transients or glitches on V CC. Figure 2 shows the maximum pulse width a negative-going V CC transient can have without causing a reset pulse. In general, as the magnitude of the transient increases, going further below the threshold, the maximum allowable pulse width decreases. Typically, for the 4.63V and 4.38V version of the MCP809/810, a V CC transient that goes 100mV below the reset threshold and lasts 20µs or less will not cause a reset pulse. A 0.1 µF bypass capacitor mounted as close as possible to the V CC pin will provide additional transient re- jection. Interfacing to µPs with Bidirectional Reset Pins Microprocessors with bidirectional reset pins, such as the Motorola 68HC11 series, can be connected to the MCP809 RESET output. To ensure a correct output on the MCP809 even when the microprocessor reset pin is in the opposite state, connect a 4.7k Ω resistor between the MCP809 RE- SET output and the µP reset pin, as shown in Figure 3. Buffer the MCP809 RESET output to other system compo- nents. 20022310 FIGURE 1. RESET Valid to V CC = Ground Circuit 20022309 FIGURE 2. Maximum Transient Duration without Causing a Reset Pulse vs. Reset Comparator Overdrive 20022311 FIGURE 3. Interfacing to Microprocessors with Bidirectional Reset I/O www.national.com 6 |
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