Interfacing to Other Voltages for
Logic Compatibility
As shown in Figure 9, the open-drain RESET output can
operate in the 2.5V to 18V range. This allows the device
to interface a µP with other logic levels.
WDI Glitch Immunity
For additional glitch immunity, connect an RC lowpass
filter as close as possible to WDI (see Figure 10).
For example, for glitches with duration of 1µs, a 12kΩ
resistor and a 47pF capacitor will provide immunity.
Layout Considerations
SRT and SWT are connected to internal precision cur-
rent sources. When developing the layout for the appli-
cation, minimize stray capacitance attached to SRT
and SWT as well as leakage currents that can reach
those nodes. SRT and SWT traces should be as short
as possible. Route traces carrying high-speed digital
signals and traces with large voltage potentials as far
from SRT and SWT as possible. Leakage currents and
stray capacitance (e.g., a scope probe, which induces
both) at these pins may cause errors in the reset and/or
watchdog timeout period. When evaluating these parts,
use clean prototype boards to ensure accurate reset
and watchdog timeout periods.
RESETIN is a high-impedance input and a high-imped-
ance resistive divider (e.g., 100kΩ to 1MΩ) sets the
threshold level. Minimize coupling to transient signals
by keeping the connections to this input short. Any DC
leakage current at RESETIN (e.g., a scope probe)
causes errors in the programmed reset threshold.
Typical Operating Circuits
RESET remains asserted as long as RESETIN is below
the regulated voltage and for the reset timeout period
after RESETIN goes high to assure that the monitored
LDO voltage is settled. Then, the µC starts operating
and triggers WDI.
If the µC fails to operate correctly (e.g., the software
execution is stuck in a loop), the WDI signal does not
trigger the watchdog timer any more, and RESET is
pulled low, resetting the µC. If the µC does not work
properly in the next loop either, the device asserts
RESET again. After three watchdog timeout periods
with no WDI falling edges, ENABLE asserts and flags
backup or safety circuits that take over the operation.
MAX16997/MAX16998
High-Voltage Watchdog Timers with
Adjustable Timeout Delay
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