The LM809 and LM810 microprocessor supervisory circuits provide a simple solution to monitor the power supplies in microprocessor and digital systems and provide a reset during power-up, power-down, and brown-out conditions. The reset signal is controlled by the factory-programmed reset threshold on the VCC supply voltage pin. When the voltage declines below the reset threshold, the reset signal is asserted and remains asserted for 240 ms after VCC rises above the threshold. The LM809 has an active-low RESET output, while the LM810 has an active-high RESET output. The available threshold options are 2.63 V, 2.93 V, 3.08 V, 4.38 V, and 4.63 V to provide precision monitoring of supply voltages.
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 4.63-V and 3.08-V options of the LM809 and LM810 use highly accurate circuitry to ensure that the reset threshold occurs only within this range (for 5-V and 3.3-V 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 3-V, 3.3-V, and 5-V nominal supply voltages.
|Reset Threshold||3 V||3.3 V||5 V|
|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%|
|2.45 ± 3%||79 – 84%||72 – 76%|
When VCC falls below 1 V, the LM809 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 100-kΩ resistor should be connected from the RESET output to ground, as shown in Figure 7.
A 100-kΩ pullup resistor to VCC is also recommended for the LM810, if RESET is required to remain valid for VCC < 1 V.
The LM809 and LM810 are relatively immune to short negative-going transients or glitches on VCC. Figure 8 shows the maximum pulse width a negative-going VCC 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 4.63-V and 4.38-V version of the LM809 or LM810, a VCC transient that goes 100 mV below the reset threshold and lasts 20 µs or less will not cause a reset pulse. A 0.1-µF bypass capacitor mounted as close as possible to the VCC pin will provide additional transient rejection.
Microprocessors with bidirectional reset pins, such as the Motorola 68HC11 series, can be connected to the LM809 RESET output. To ensure a correct output on the LM809 even when the microprocessor reset pin is in the opposite state, connect a 4.7-kΩ resistor between the LM809 RESET output and the µP reset pin, as shown in Figure 9. Buffer the LM809 RESET output to other system components.
When VCC supply voltage declines below the reset threshold, the RESET output is asserted. For LM809, the active-low RESET output is low. For LM810, the active-high RESET output is high.
When the VCC supply voltage rises above the reset threshold, the RESET output resets after 240 ms. For LM809, the active-low RESET output rises high. For LM810, the active-high RESET output drops low.