
Detailed Description
RESET
/RESET Output
A microprocessor
’
s (μP
’
s) reset input starts the micro-
processor in a known state. The MXD1810
–
MXD1813/
MXD1815
–
MXD1818 μP supervisory circuits assert
reset to prevent code-execution errors during power-
up, power-down, and brownout conditions (Figure 4).
Whenever V
CC
falls below the reset threshold, the reset
output asserts. Once V
CC
exceeds the reset threshold,
an internal timer keeps the reset output asserted for the
specified reset timeout period (t
RP
). Reset is also trig-
gered by an externally initiated rising edge on the
RESET
pin (MXD1813/MXD1818), following a low signal
of 1.5μs minimum duration.
Push-Button Reset (MXD1813/MXD1818)
Many μP-based products require push-button reset
capability (Figure 5), allowing the operator, a test tech-
nician, or external logic circuitry to initiate reset. On the
MXD1813/MXD1818, a logic-low on
RESET
held for
greater than 1.5μs asserts a reset.
RESET
deasserts
following a 100ms minimum reset timeout delay
(t
PBRST
). A manual reset input shorter than 1.5μs may
release
RESET
without the 100ms minimum reset time-
out delay. To facilitate use with mechanical switches,
the MXD1813/MXD1818 contain internal debouncing
circuitry. A debounced waveform is shown in Figure 6.
Applications Information
Interfacing to μPs with Bidirectional
Reset Pins
Since the
RESET
output on the MXD1811/MXD1816 is
open drain, these devices interface easily with μPs that
have bidirectional reset pins, such as the Motorola
68HC11. Connecting the μP supervisor
’
s
RESET
output
directly to the microcontroller
’
s (μC
’
s)
RESET
pin allows
either device to assert reset (Figure 7). No external
pullup resistor is required, as it is contained within the
MXD1811/MXD1816.
Negative-Going V
CC
Transients
In addition to issuing a reset to the μP during power-up,
power-down, and brownout conditions, these devices
are relatively immune to short-duration, negative-going
V
CC
transients (glitches).
M
Low-Power μP Reset Circuits in 3-Pin
SC70/SOT23
_______________________________________________________________________________________
5
PIN
SC70
SOT23
NAME
FUNCTION
2
1
RESET
Push-Pull, Active-High Reset Output. RESET changes from low to high when V
CC
drops
below the selected reset threshold. RESET remains high for the reset timeout period after
V
CC
exceeds the device reset threshold.
Supply Voltage and Input for Reset Threshold Monitor
Ground
1
3
2
3
V
CC
GND
MXD1812/MXD1817
PIN
SC70
SOT23
NAME
FUNCTION
2
1
RESET
Open-Drain, Active-Low Reset Output with Manual Reset Detect.
RESET
changes from
high to low when V
CC
drops below the selected reset threshold, or
RESET
is externally
pulled low for at least 1.5μs.
RESET
remains low for the reset timeout period after V
CC
exceeds the device reset threshold or after the external manual reset is released.
RESET
has an internal 5.5k
pullup resistor.
1
3
2
3
V
CC
GND
Supply Voltage and Input for Reset Threshold Monitor
Ground
MXD1813/MXD1818
Pin Descriptions (continued)