L9678P datasheet(148/203 Pages) STMICROELECTRONICS

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Safing logic

L9678P, L9678P-S

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sometimes this is due to the amount of data to be transmitted. A simplified diagram shows

the basic communication differences of in and out of frame responses.

Figure 46. In-frame example

Figure 47. Out of frame example

Synchronization between clock domains relies upon inter-frame gap.

10.4

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 ARM1INT and ARM2INT. As previously stated, there is a

maximum of 4 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.

10.4.1

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 the same as

or different from 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. The ADD_VAL and SUB_VAL programmed values are the same for all

safing sources.

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

updated. Each event counter is then 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.

POS_COUNT and NEG_COUNT are not updated if microcontroller stops reading SAF_CC

bits (this must be avoided otherwise ARMING set and reset will not be possible).

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