L9001 datasheet(24/38 Pages) STMICROELECTRONICS

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Watchdog

L9001

24/38

DS12089 Rev 2

Watchdog

The device hosts a watchdog control unit. The μC must trigger the watchdog with a toggle

on WDI pin within a configurable time period tWDT (also called normal window) which

depends on external capacitance at pin WDT and is specified as tWDT(Cwdt). The feedback

of the watchdog control can be checked monitoring output pin WDO.

Accuracy of the window (including internal spread) is shown in the table below with

reference to the case a 47 nF cap is mounted:

tWDT(Cwdt) = tWDT(47 nF) * Cwdt/47 nF

As soon as L9001 is in ACTIVE mode and resets of the VDD regulators are high (only the

enabled regulators are considered), the watchdog is switched on and opens a long window

which is 10 times the normal window. If the watchdog is correctly served before the end of

this window, subsequent normal windows are opened as described in Figure 6.

In order to serve the watchdog, WDI must toggle from low state to high state. A correct

toggle is recognized when high state at WDI lasts as long as tWDI_h and low state as short

as tWDI_l; when there is a correct WDI toggle, a WDI internal flag is set. At the end of the

timing window tWDT(Cwdt), the flag is checked and if it is set to 1, the system is working

properly, the flag is reset to 0 and the cycle goes on. If the flag is still at 0, a watchdog failure

is generated. It follows that more than one WDI toggle can occur in the time window.

WDO is an open drain output (low active). In case of watchdog failure the output WDO is

driven low for the specified time tWDO; the output WDO will be relased after this time unless

the device enters latch mode (see Section 8.2 and Figure 6 for details).

Watchdog failure is also counted as failure condition for LATCH mode enter after 4

consecutive failures as explained in Section 8.

In software development mode (FLASH mode) watchdog is disabled. This can be done by

forcing WDT>WDT_thi. The device can regain Active mode if WDT<WDT_tho; the function

is also switched off when entering LOW POWER mode, and is switched on again when re-

entering ACTIVE mode (see Section 8.1 for details)

When WDT pin is connected to a fixed voltage, watchdog functionality is frozen, but

regulators still function normally.

Table 14. Watchdog parameters

Symbol

Parameter

Test condition

Min.

Typ.

Max.

Unit

tWDT(47nF)

Watchdog timer window length

reference working point

tWDT with C(WDT) = 47 nF

tWDT_lw(47nF) Watchdog timer long window

tWDT_lw with C(WDT) = 47 nF

350

400

450

tWDO

WDO active pulse duration

Cwdt

WDT pin capacitor

Application info

100

tWDI_h

Minimum high level WDI duration

to trigger WD

μs

tWDI_l

Minimum low level WDI duration

μs

WDI_h

WDI input high level threshold

1.5

WDI_l

WDI input low level threshold

1.05

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