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http://www.union-ic.com

Rev.04 Dec.2013

8/15

UM809/810

flow to ground, holding RESET

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low (Figure 8). R1’s value is not critical; 100kΩ is large enough

not to load RESET

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and small enough to pull RESET

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to ground.

Figure 8. RESET

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Interfacing to μPs with Bidirectional Reset Pins

Some µP’s (such as Motorola 68HC11) have bi-directional reset pins. Depending on the current

drive capability of the processor pin, an indeterminate logic level may result in series with the

output of the UM809 (Figure 9). If there are other components in the system which requires a

reset signal, they should be buffered so as not to load the reset line. If the other components are

required to follow the reset I/O of the μP, the buffer should be connected as shown with the solid

line.

Figure 9. Interfacing to Bidirectional Reset I/O

Benefits of Highly Accurate Reset Threshold

Most µP supervisor circuits have 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.

The UM8_ L/T/Z use highly accurate circuitry to ensure that reset is asserted close to the 5% limit,

and long before the supply has declined to 10% below nominal.