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AS1910V-T

Dual-Voltage Microprocessor Supervisory Circuits with Manual Reset and Watchdog

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AS1910 - AS1915

Data Sheet - Application Information

9 Application Information

Watchdog Input Current

The watchdog input is driven through an internal buffer and an internal series resistor from the watchdog timer (see

Figure 11 on page 9). When pin WDI is left unconnected (watchdog disabled), the watchdog timer is serviced within the

watchdog timeout period (see tWD on page 6) by a low-high-low pulse from the counter chain. For minimum watchdog

input current (minimum overall power consumption), pull WDI low for most of the watchdog timeout period, pulsing it

low-high-low once within the first 7/8 (87.5%) of the watchdog timeout period to reset the watchdog timer.

Note: If WDI is externally driven high for the majority of the timeout period, up to 160µA can flow into pin WDI.

Interfacing to Bi-Directional CPU Reset Pins

Since the reset outputs of the AS1912 and AS1915 are open drain, these devices interface easily with processors that

have bi-directional reset pins. Connecting the processor reset output directly to the AS1912/AS1915 RESETN pin with

a single pullup resistor (see Figure 14) allows the AS1912/AS1912 to assert a reset.

Figure 14. AS1912 or AS1915 RESETN-to-CPU Bi-Directional Reset Pin

VCC

VCC

CPU

RESETN

GND

1

RESETN

VCC 6

AS1912/

AS1915

Reset

Generator

GND 2

Fast Negative-Going Transients

Fast, negative-going VCC transients normally do not require the CPU to be shutdown. The AS1910 - AS1915 are virtu-

ally immune to such transients. Resets are issued to the CPU during power-up, powerdown, and brownout conditions.

Note: VCC transients that go 100mV below the reset threshold and last ≤ 55µs typically will not assert a reset pulse.

Valid Reset to VCC = 0

The AS1910 - AS1915 are guaranteed to operate properly down to VCC = 1V. For applications requiring valid reset lev-

els down to VCC = 0, a pulldown resistor to active-low outputs (push/pull only) and a pullup resistor to active-high out-

puts (push/pull only) will ensure that the reset line is valid during the interval where the reset output can no longer sink

or source current.

Watchdog Tips

Careful consideration should be taken when implementing the AS1910 - AS1915 watchdog feature.

One method of supervising software code execution is to set/reset the watchdog input at different places in the code,

rather than pulsing the watchdog input high-low-high or low-high-low. This method avoids a loop condition in which the

watchdog timer would continue to be reset inside the loop, preventing the watchdog from ever timing out.

Figure 15 shows a flowchart where the input/output driving the watchdog is set high at the beginning of the routine, set

low at the beginning of every subroutine, then set high again when the routine returns to the beginning. If the routine

should hang in a subroutine, the problem would quickly be corrected, since the I/O is continually set low and the watch-

dog timer is allowed to time out, causing a reset or interrupt to be issued (see Watchdog Input Current on page 11).

This method results in higher averaged WDI input current over time than a case where WDI is held low for the majority

(87.5%) of the timeout period and periodically pulsing it low-high-low.

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