L9678P datasheet(152/203 Pages) STMICROELECTRONICS

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Safing logic

L9678P, L9678P-S

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DocID029274 Rev 3

10.6

Arming pulse stretch

Upon a valid command processed by the safing logic, the Dwell bits to stretch the arming

time assertion (dwell time) apply to each safing record and is used to help safe the

deployment sequence to avoid undesired behaviour.

Once dwell time has started, it will continue, regardless of the En (Record Enable) bit. Dwell

will be truncated in case of SSM reset. Dwell values in the safing records are transferred to

the ARM signal. A dedicated counter is designed for ARM output pin. If different dwell

values are assigned to ARM, the longer value is used. Dwell times can only be extended,

not reduced. If the remaining dwell time is less than the new dwell extension setting, the

new setting will be loaded into the dwell counter.

Dwell times are user programmable.

The behavior of the pulse stretch timer is shown in Figure 51.

Figure 51. Pulse stretch timer example

10.7

Additional communication line

The ACL pin is the Additional Communication Line input that provides a means of safely

activating the arming outputs (ARM and VSF) for disposal of restraints devices at the end of

vehicle life.

A valid ACL detection (as described below) allows L9678P to transition from Scrap state to

Arming state. To remain in Arming state L9678P must receive the correct ACL signal; this

must occur before the scrap time-out timer expires (TdisEOL).

While the System Operating State Machine is in Arming state, the arming outputs are

asserted (ARM=1, VSF on). If the ACL is not correctly received before the time-out expires,

the System Operating State Machine reverts back to the Scrap state, and the arming

outputs are deactivated.

L9678P 데이터시트(PDF) 152 Page - STMicroelectronics