L9679P datasheet(164/279 Pages) STMICROELECTRONICS

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Deployment drivers

L9679P

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DS11627 Rev 7

Short between loops diagnostics

In case the previous test has reported a leakage to ground fault, the short between loops

diagnostics shall be run. The same procedure is followed as described for normal leakage

tests except the fact that in this case all the pull-down current generators have to be

deactivated (not only the one for the pin under test), by means of the PD_CURR bit in the

Diagnostic Request Register (LPDIAGREQ). If a leakage or ground fault is not present, then

the channel under test has a short to another squib/pyroswitch loop.

The condition of two open channels, i.e. without squib/pyroswitch resistance connecting

SFx to SRx, that have a short between loops on SFx cannot be detected. If only one of the

two shorted SFx pins is open, the fault will be indicated on the open channel.

Squib/pyroswitch resistance measurement

During a resistance measurement, a two-step process is performed. At the first step, both

ISRC current generator and ISNK current limited switch are enabled and connected to the

selected SFx and SRx channels, through ISRC, ISRC_CURR_SEL, ISNK and

RES_MEAS_CHSEL bit fields in the Loop Diagnostic Request Register (LPDIAGREQ). The

ISRC current can be configured to either 40 mA or 8 mA nominal value through the

ISRC_CURR_SEL bit in the LPDIAGREQ register providing the user with two different

measurement range options. A differential voltage is created between the SFx and SRx pin

based on the ISRC current and squib/pyroswitch resistance between the pins. The SPI

interface will provide the first resistance measurement voltage (Vdiff1) based on the

amplifying factor of the differential amplifier and a 10-bit internal ADC conversion. The

second measurement step (bypass measurement) is performed redirecting ISRC to the

selected SRx pin, while keeping ISNK on; this way, the differential amplifier and following

ADC will output the offset measurement through SPI (Vdiff2). Microcontroller is then allowed

to calculate the mathematical difference between first and second measurements to obtain

the real squib/pyroswitch resistance value.

(assuming RLKG_SF >> RSQ)

Table 10. Short between loops diagnostics decoding

Fault condition on

squib/pyroswitch

channel

Channel leakage diagnostics with

PD_CURR on (for other channels

than the one under test)

Channel leakage diagnostics

with PD_CURR off

(for all channels)

No shorts

No fault

Short to battery

STB fault

Short to ground

STG fault

Short between loops

STG fault

No fault

Vdiff1 GRSQ ISRC_*

RLKG_SF RSQ



RLKG_SF RSQ

--------------------------------------------













RSQ

RLKG_SF RSQ

-------------------------------------------- VLKG_SF VSRx_RM

–







+ GRSQ Voff_RSQ



Vdiff2

GRSQ RSQ



RLKG_SF RSQ

--------------------------------------------

VLKG_SF VSRx_RM

–







GRSQ Voff_RSQ



RSQ

Vdiff1 Vdiff2

–

GRSQ ISRC_*



-----------------------------------------

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