UM809_15 Datasheet, PDF (7/14 Pages) Union Semiconductor, Inc. – 3-Pin Microprocessor Reset Circuits

UM809/810
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enough not to load RESET and small enough to pull RESET to ground.
A 100kΩ pull-up resistor to V_CC is also recommended for the UM810 if RESET is required to
remain valid for V_CC < 1V.
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Figure 10. RESET Valid to V_CC=Ground Circuit
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 11). 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.
( )* for UM810
Figure 11. 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.
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