________________________________________________________________________

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

8/14

UM805/811/812

high.

_____________

goes low. Any time

_____________

goes low. The

_____________

remains low for the

reset timeout period.

The manual reset input (MR

_______

) can also initiate a reset. See the Manual Reset Input section.

The UM812 has an active-high RESET output that is the inverse of the UM805/811’s RESET

_____________

output. The UM805 uses an open-drain output, and the UM811/812 have a push-pull output stage.

Connect a pull-up resistor on the UM805’s RESET

_____________

output to any supply between 0 and 6V.

Manual Reset Input

Many μP-based products require manual reset capability, allowing the operator, a test technician, or

external logic circuitry to initiate a reset. A logic low on MR

_______

asserts reset. Reset remains asserted

while MR

_______

_______

returns high. This input

has an internal 20kΩ pull-up resistor, so it can be left open if it is not used. MR

_______

can be driven with

TTL or CMOS-logic levels, or with open-drain/collector outputs. Connect a normally open

momentary switch from MR

_______

to GND to create a manual-reset function; external debounce

circuitry is not required. If MR

_______

is driven from long cables or if the device is used in a noisy

environment, connecting a 0.1μF capacitor from MR

_______

to ground provides additional noise

immunity.

Reset Threshold Accuracy

The UM805/811/812 are ideal for systems using a 5V±5% or 3V±5% power supply with ICs

specified for 5V±10% or 3V±10%, respectively. They are designed to meet worst-case

specifications over temperature. The reset is guaranteed to assert after the power supply falls out of

regulation, but before power drops below the minimum specified operating voltage range for the

system ICs. The thresholds are pre-trimmed and exhibit tight distribution, reducing the range over

which an undesirable reset may occur.