L9396 datasheet(75/109 Pages) STMICROELECTRONICS

75 / 109 page

DS12539 Rev 10

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L9396

System functional safety implementations

109

Figure 17. WD1 state machine

Depending on the value of the WD_CNT counter the device will stop the drivers, will send

the WD1_RESET or will enable the drivers.

The WD_CNT will be incremented by a number of steps as defined through the SPI

configurable parameter WD_cnt_good_step each time a correct answer is given in the right

time interval or a Query arrives in the right time interval. In all the other cases, as defined in

the WD state machine, the WD_CNT will be decremented by a number of steps as defined

through the SPI configurable parameter WD_cnt_bad_step.

If WD_CNT reaches the value of zero two different behaviors are possible depending on the

value of WD_RST_EN. If WD_RST_EN is set to 1 then a WD_RST will be sent by the

device and the flag WD_RST_CNT will be set; else if WD_RST_EN is set to 0 then the

WD_RST will not be sent by the device but the flag WD_RST_CNT will be set.

Two different thresholds are defined (both programmable through SPI): WD_th_low and

WD_th_high.

If WD_CNT value is lower than WD_th_low, but greater than zero, the drivers are disabled

such as any WD_RST. If WD_CNT is greater than WD_TH_LOW and lower than

WD_TH_HIGH the load actuation is managed in hysteresis mode:

If drivers are ON it will be stopped only when WD_CNT becomes lower than WD_TH_LOW,

while if actuation was OFF it will be performed only when WD_CNT becomes equal to

WD_TH_HIGH and only when WD_CNT exceeds this threshold drivers are activated.

WD_RST

Request = No Req

t = T_Req_TimeOut

WD_CNT--

then WD_CNT++

Request = YES

t < T_Valid_Req_Start

REQ 1

WAIT

Request = YES

T_Valid_Req_Start < t < T_Valid_Req_End

WD_CNT--

Request = YES

T_Valid_Req_End < t < T_Req_TimeOut

Request = No Req

t = T_Valid_Req_End

Request = No Req

t = T_Valid_Req_Start

REQ 3

WAIT

REQ 2

WAIT

1 Seed Request

ANSWER 1

WAIT

IDLE

WD_CNT--

Answer = YES

T_Valid_Answ_End < t < T_Answ_TimeOut

WD_CNT--

Answer = YES

t < T_Valid_Answ_Start

elseif (Answer==wrong) then WD_CNT--

If (Answer==correct) then WD_CNT++

Answer = YES

T_Valid_Answ_Start < t < T_Valid_Answ_End

Answer = No Answer

t = T_Valid_Answ_Start

WD_RST

Answer = No Answer

t = T_Answ_TimeOut

Answer = No Answer

t = T_Valid_Answ_End

ANSWER 2

WAIT

ANSWER 3

WAIT

GADG2401171444PS

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