Rainbow Electronics MAX6869 - Manual (Page 12)

MAX6854–MAX6869
Transient Immunity
In addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, the MAX6854–
MAX6869 are relatively immune to short-duration supply
transients, or glitches. The Maximum Transient Duration
vs. Reset Threshold Overdrive graph in the Typical
Operating Characteristics shows this relationship.
The area below the curve of the graph is the region in
which these devices typically do not generate a reset
pulse. This graph was generated using a falling pulse
applied to V
CC
, starting 100mV above the actual reset
threshold, V TH , and ending below this threshold (reset-
threshold overdrive). As the magnitude of the transient
increases, the maximum allowable pulse width
decreases. Typically, a 100mV V
CC transient duration
of 40µs or less does not cause a reset.
Interfacing to Other Voltages for Logic
Compatibility
The open-drain RESET output can be used to interface
to a µP with other logic levels. As shown in
Figure 4, the
open-drain output can be connected to voltages from 0
to 5.5V.
Generally, the pullup resistor connected to RESET con-
nects to the supply voltage that is being monitored at
the IC’s V
CC
input. However, some systems use the
open-drain output to level-shift from the monitored sup-
ply to reset circuitry powered by another supply voltage.
Keep in mind that as the supervisor’s V
CC decreases,
so does the IC’s ability to sink current at RESET.
Ensuring a Valid
RESET
Down to V
CC
= 0V
(Push-Pull
RESET
)
When V
CC
falls below 1.1V, RESET’s current-sinking
capability declines drastically. The high-impedance
CMOS logic inputs connected to RESET can drift to
undetermined voltages. This presents no problems in
most applications, since most µPs and other circuitry
do not operate with V
CC below 1.1V.
In those applications where RESET must be valid down
to 0, add a pulldown resistor between RESET and GND
for the MAX6854/MAX6857/MAX6858/MAX6861/
MAX6864/MAX6867 push-pull outputs. The resistor sinks
any stray leakage currents, holding RESET low (
Figure
5). Choose a pulldown resistor that accommodates leak-
ages, such that RESET is not significantly loaded and is
capable of pulling to GND. The external pulldown cannot
be used with the open-drain reset outputs.
Watchdog Software Considerations
One way to help the watchdog timer monitor software
execution more closely is to set and reset the watchdog
Nanopower µP Supervisory Circuits with
Optional Manual Reset, Watchdog Timer
12 ______________________________________________________________________________________
MAX6856
MAX6859
MAX6860
MAX6863
MAX6866
MAX6869
GND
N
RESET
RESET
1.8V
V CC
V CC
GND
3.3V
µP
100kΩ
Figure 4. Interfacing with Other Voltage Levels
V CC
V
CC
2MΩ
MAX6854
MAX6857
MAX6858
MAX6861
MAX6864
MAX6867
GND
RESET
Figure 5. Ensuring RESET Valid to VCC = Ground