L9679P datasheet(199/272 Pages) STMICROELECTRONICS

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L9679P

Safing logic

271

Figure 56. In-frame example

At least one bit needed to allow for synchronization between clock domains (SPI clock and

system clock).

Out-of-frame example:

Figure 57. Out-of-frame example

Synchronization between clock domains relies upon inter-frame gap.

12.4

Safing state machine operation

State machine operation is disabled when the safing state machine reset signal is active as

described in the power supply diagnostics and controls section of this document. The

outputs of the state machine are ARMxREQ. As previously stated, there is a maximum of 16

safing records available to the state machine. Inputs to the safety state machine are

programmed safing records and sensor data. The configuration of the state machine is

common to all sensors.

12.4.1

Simple threshold comparison operation

In this mode, sensor data received through the sensor SPI interface and validated by the

safing record is passed to the safing algorithm. The simple threshold comparison algorithm

compares the received data to two thresholds, SAF_TH (positive threshold) and (-SAF_TH)

(negative threshold). If the sensor data is greater than SAF_TH or is less than (-SAF_TH)

then and event is flagged and the event counter is incremented based on the programmed

value of ADD_VAL. If sensor data does not trigger the SAF_TH comparators, the counter is

decremented by SUB_VAL. SUB_VAL is programmed by the user and can be same or

different than ADD_VAL. This feature allows for an asymmetrical counter function making

the system either more or less sensitive to sensor data. Since sensor data can indicate a

positive or negative event, the algorithm maintains separate event counters, POS_COUNT

and NEG_COUNT. ADD_VAL and SUB_VAL programmed values are the same for both

event counters.

On each sensor sample, the event counters, POS_COUNT and NEG_COUNT, are updated

based on the SAF_TH comparators. Likewise, each event counter is compared with a

corresponding arming threshold. In this case, POS_COUNT value is compared to

ARMP_TH and NEG_COUNT to ARMN_TH. ARMP_TH and ARMN_TH are programmable

thresholds set by the user. The compared result will set ARMP and ARMN to either ‘1’ or ‘0’

depending on the comparison status. If ARMP_TH or ARMN_TH are set to 0, the arming will

be activated immediately entering in safing state.

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