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AS1916

Microprocessor Supervisory Circuits with Manual Reset and Watchdog

austriamicrosystems AG 

AS1916 - AS1918

Datasheet - Application Information

9 Application Information

9.1 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 8). When

pin WDI is left unconnected (watchdog disabled), the watchdog timer is serviced within the watchdog timeout period (see tWD on page 5) 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.

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

9.2 Interfacing to Bi-Directional CPU Reset Pins

a Since the reset output of the AS1918 is open drain, this device interfaces easily with processors that have bi-directional reset pins. Connecting

the processor reset output directly to the AS1918 RESETN pin with a single pullup resistor (see Figure 12) allows the AS1918 to assert a reset.

v Figure 12. AS1918 RESETN-to-CPU Bi-Directional Reset Pin

ill VCC

t VCC

AG t s CPU

RESETN

ms ten GND

VCC 5

1

RESETN

AS1918

Reset

Generator

GND 2

a on 9.3 Fast Negative-Going Transients

c Fast, negative-going VCC transients normally do not require the CPU to be shutdown. The AS1916 - AS1918 are virtually immune to such

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

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

a 9.4 Valid Reset to VCC = 0

ic The AS1916 - AS1918 are guaranteed to operate properly down to VCC = 1V.

For AS1916 and AS1917 applications requiring valid reset levels down to VCC = 0, a pulldown resistor to active-low outputs and a pullup resistor

n to active-high outputs will ensure that the reset line is valid during the interval where the reset output can no longer sink or source current.

h 9.5 Watchdog Tips

Careful consideration should be taken when implementing the AS1916 - AS1918 watchdog feature.

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

e 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

Tthe loop, preventing the watchdog from ever timing out.

Figure 13 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 watchdog timer is allowed to time out, causing a reset or interrupt to be issued

(see Watchdog Input Current on page 9). 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.

www.austriamicrosystems.com/Supervisors/AS1916

Revision 1.04

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