AS1916 Datasheet, PDF (8/13 Pages) ams AG – Microprocessor Supervisory Circuits with Manual Reset and Watchdog

AS1916 - AS1918
Data Sheet
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Watchdog Input
Watchdog Input
The integrated watchdog feature can be used to monitor processor activity via pin WDI, and can detect pulses as short
as 50ns. The watchdog requires that the processor toggle the watchdog logic input at regular intervals, within a speci-
fied minimum timeout period (1.5s, typ). A reset is asserted for the reset timeout period. As long as reset is asserted,
the timer remains cleared and is not incremented. When reset is deasserted, the watchdog timer starts counting
(Figure 11).
Note: The watchdog timer can be cleared with a reset pulse or by toggling WDI.
Figure 11. Watchdog Timing Relationship
VCC
tRST
RESETN
tRP
WDI
tWD
tRP
The RESET signal is the inverse of the RESETN signal.
The watchdog is internally driven low during most (87.5%) of the watchdog timeout period (see tWD on page 4) and
high for the rest of the watchdog timeout period. When pin WDI is left unconnected, this internal driver clears the
watchdog timer every 1.4s. When WDI is tri-stated or is not connected, the maximum allowable leakage current is
10µA and the maximum allowable load capacitance is 200pF.
Note: The watchdog function can be disabled by leaving pin WDI unconnected or connecting it to a tri-state output
buffer.
Manual Reset Input
The active-low pin MRN is used to force a manual reset. This input can be driven by CMOS logic levels or with open-
drain collector outputs.
Pulling MRN low asserts a reset which will remain asserted as long as MRN is kept low, and for the timeout period (see
tRP on page 3) after MRN goes high (140ms min). The manual reset circuitry has an internal 50kΩ pullup resistor, thus
it can be left open if not used.
To create a manual-reset circuit, connect a normally open momentary switch from pin MRN to GND (see Figure 1 on
page 1); external debounce circuitry is not required in this configuration.
If MRN is driven via long cables or the device is used in a noisy environment, a 0.1µF capacitor between pin MRN and
GND will provide additional noise immunity.
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Revision 0.52
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