L4969UR-E datasheet(34/46 Pages) STMICROELECTRONICS

34 / 46 page

Control and status registers

L4969UR-E, L4969URD-E

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Doc ID 022587 Rev 2

4.5.4

Wakeup watchdog

Figure 21.

Wakeup watchdog

If the Timer is activated during Normal mode by setting WDEN in WDC, an “acknowledge-

free” sequence is started for a predefined programmable time. Window Watchdog activity is

resumed after expiry of the timer.

To be able to detect the timeout, the corresponding interrupt enable must be set in GIEN.

This mode can also be used to allow a bootstrap loader mode with longer execution times

than the maximum specified window. Correct startup of this loader is safely detected upon

missing response following the timeout.

Note:

Special considerations for the timer restart via WDC write:

Due to a restriction in the transition from Wake-up Watchdog to Window Watchdog an

unwanted low pulse on RESET (Window Watchdog failure) can be triggered when WDC

Therefore the timer can only be restarted by rewriting WDEN twice in WDC when the

location of the timer expiration is considered.

This is the case, when the expiration of the timer is monitored through timer expiration

interrupt via NINT (configuration as in Figure 21). Here a safe rewrite to the WDC register is

possible directly after this event has been detected (the time for event processing plus the

duration of the corresponding SPI frame are far longer than the Wake-up Watchdog to

Window Watchdog state transition).

When the timer expiration cannot be known while updating the WDC register, two strategies

are possible to bypass this behaviour:

Disable the Window Watchdog function as described in section 4.5, in the Watchdog

control register to avoid a false Window Watchdog failure. The potential impact on a

safe application supervision has to be considered.

Access the internal state of the Wake-up Watchdog to identify a safe window for a WDC

rewrite (see Figure 22):

the internal state of the Wake-up Watchdog prescaler can be accessed via NINT after

setting the bit D6, ‘CG0’ of the RCADJ register.

To avoid that other active interrupt sources pull NINT low they have to be masked by

clearing the global interrupt mask register GIEN.

An expiration of the Wake-up Watchdog can only occur with the rising edge of the

rectangular waveform now visible on NINT, so that a safe rewrite of the WDC register can

take place at any time while NINT is high or directly after the falling edge. After WDC rewrite

Window Wd

Timer (80ms .. 45min)

NINT

Ack Window &

Timeout and resume Window Wd

Interrupt active upon timeout (via GIEN)

Start Timer

restart timer

safely before expiration

by writing WDC twice

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