L9678P datasheet(138/215 Pages) STMICROELECTRONICS

138 / 215 page

Watchdog timer

L9678P, L9678P-S

138/216

DS11626 Rev 9

Following the description of the above states:

A single SPI command (WD1_TEST) is used to activate test states for the watchdog

circuitry.

Table 9. Watchdog timer status description

State/Signal

Description

WD1 INITIAL

Default state entered from startup. While in this state, no watchdog service is

required, and the IC may stay in this state indefinitely. For system safety, all

arming signals are disabled during this state to prevent deployment.

WD1 RUN

Normal run time state where WD1 service is required.

WD1 TEST

A special state used to test the watchdog function. Normally, this state will only

be checked once per power cycle by the software, but there is no inherent

restriction in the watchdog logic preventing periodic testing. This state allows

testing of the watchdog without setting WD1_LOCKOUT=1, which can only be

cleared via WSM reset. Deployment is inhibited when the WD state machine is

in this state.

WD1 RESET

State entered when a WD1_ERROR occurs. This is a timed-duration state that

is automatically exited after 1ms.

WD1 OVERRIDE

A special state used to disable watchdog functionality for development

purposes. Other logic within the IC can use this state to emulate the WD1 RUN

state without the need to service WD1.

WSM_RESET

Signal used to reset the WD1 state machine to the WD1 INITIAL state and all

signals to their inactive values

WD1_refresh OK Signal that is asserted only if the watchdog is refreshed ('A' - 'B' or 'B' - 'A' seq.)

within the WD1 time window

WD1_ERROR

Signal that is asserted if the watchdog refresh fails to occur during the WD1

time window.

WD1_WDR

Watchdog Reset – latched signal that is activated whenever a watchdog error

is qualified. For WD1, this occurs when WD1 service is required, but not

received. This signal is SPI-readable.

WD1_TM

Test Mode – a signal that indicates that WD1 is being tested. This signal is

SPI-readable.

WD1_LOCKOUT

A latched signal activated if an unexpected WD1 error occurs. This signal is

permanently latched when set (until WSM_RESET). When set, all arming

signals are disabled, preventing deployment. This signal is SPI-readable.

SPI_WD1_TEST

SPI command used to enter WD1 TEST state from WD1 RUN state, or to enter

WD1 OVERRIDE state from INITIAL state if WDT/TM pin voltage is greater

than the threshold. This command has no effect in other states.

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