Datasheet
STWD100 Operation
Doc ID 14134 Rev 7 7/25
2 Operation
The STWD100 device is used to detect an out-of-control MCU. The user has to ensure
watchdog reset within the watchdog timeout period, otherwise the watchdog output is
asserted and MCU is restarted. The STWD100 can be also enabled or disabled by the
chip enable pin.
2.1 Watchdog input (WDI)
The WDI input has to be toggled within the watchdog timeout period, t
WD
, otherwise the
watchdog output, WDO
, is asserted. The internal watchdog timer, which counts the t
WD
period, is cleared either:
1. by a transition on watchdog output, WDO
(see Figure 8 on page 12) or
2. by a pulse on enable pin, EN
(see Figure 10 on page 14) or
3. by toggling WDI input (low-to-high on all versions and high-to-low on STWD100xW,
STWD100xX and STWD100xY only).
The pulses on WDI input with a duration of at least 1 µs are detected and glitches shorter
than 100 ns are ignored.
If WDI is permanently tied high or low and EN
is tied low, the WDO toggles every 3.4 ms
(t
WD
) on STWD100xP and every t
WD
and t
PW
on STWD100xW, STWD100xX and
STWD100xY (see Figure 8 on page 12).
2.2 Watchdog output (WDO)
When the V
CC
exceeds the timer startup voltage V
START
after power-up, the internal
watchdog timer starts counting. If the timer is not cleared within the t
WD
, the WDO will go
low (see Figure 6).
After exceeding the t
WD
, the WDO is asserted for t
PW
on STWD100xW, STWD100xX and
STWD100xY regardless of possible WDI transitions (see Figure 9 on page 13). On
STWD100xP WDO
is asserted for a minimum of 10 µs and a maximum of t
WD
after
exceeding the t
WD
period (see Figure 8 on page 12 and Figure 9 on page 13).
The STWD100 has an active low open drain or push-pull output. An external pull-up resistor
connected to any supply voltage up to 6 V is required in case of open drain WDO
output
(see Figure 4). Select a resistor value large enough to register a logic low, and small enough
to register a logic high while supplying all input current and leakage paths connected to the
reset output line. A 10 kΩ pull-up resistor is sufficient in most applications.