UM803JS datasheet(6/11 Pages) UNIONSEMI

6 / 11 page

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http://www.union-ic.com Rev.01 Jan.2015

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UM803

Detailed Description

A microprocessor’s (µP’s) reset input starts the µP in a known state. The UM803 asserts reset to

prevent code-execution errors during power-up, power-down, or brownout conditions. It asserts a

reset signal whenever the VCC supply voltage declines below a preset threshold, keeping it asserted

for at least 140ms after VCC has risen above the reset threshold. The UM803 uses an open-drain

output. Connect a pull-up resistor on the UM803’s RESET

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output to any supply between 0 and 6V.

Applications Information

Negative-Going VCC Transient

In addition to issuing a reset to the µP during power-up, power-down, and brownout conditions, the

UM803 is relatively immune to short-duration negative-going VCC transients (glitches).

Figure 1 shows typical transient duration vs. reset comparator overdrive, for which the UM803 does

not generate a reset pulse. The graph was generated using a negative-going pulse applied to VCC,

starting 0.5V above the actual reset threshold and ending below it by the magnitude indicated (reset

comparator overdrive). The graph indicates the maximum pulse width a negative-going VCC

transient can have without causing a reset pulse. As the magnitude of the transient increases (goes

farther below the reset threshold), the maximum allowable pulse width decreases. Typically, for the

UM803L_, UM803M_ and UM803J_, a VCC transient that goes 100mV below the reset threshold

and lasts 20µs or less will not cause a reset pulse. A 0.1µF bypass capacitor mounted as close as

possible to the VCC pin provides additional transient immunity.

Figure 1. Maximum Transient Duration without Causing a Reset Pulse vs.

Reset Comparator Overdrive

Interfacing to μPs with Bidirectional Reset Pins

Since the RESET

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output on the UM803 is open drain, this device interfaces easily with μPs that

have bidirectional reset pins, such as the Motorola 68HC11. Connecting the μP supervisor’s

RESET

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output directly to the microcontroller’s (μC’s) RESET

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pin with a single pull-up resistor

UM803L_/M_/J_

UM803T_/S_/R_/Z_

UM803JS Datasheet(PDF) 6 Page - Union Semiconductor, Inc.