L9396 datasheet(76/109 Pages) STMICROELECTRONICS

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System functional safety implementations

L9396

76/109

DS12539 Rev 6

In this way, activation of the drivers can be performed only if the watchdog has been started

and a certain number of good Request/Answer has been exchanged.

Figure 18. WD1_RESET & DRIVERS ENABLE versus WD_CNT value

All the status information is stored into the WD_Status_reg, readable through SPI. This

is sent to the microcontroller, the device restarts the WD state machine in IDLE mode

waiting for the seed request from microcontroller through SPI. This is valid also in case

PRUN WD sends a reset to µC. WD configuration registers are preserved, but can be

modified by the microcontroller before the WD mechanism has started. In the same way,

also the status register, WD_Status_reg, is preserved and can be read through SPI.

Two cases of unexpected errors have been identified:



If a request of a new seed arrives to the device before the previous answer is received,

the device will serve the new request, sending the old seed decreasing the value of

WD_CNT by the amount WD_cnt_bad_step.



If an answer arrives to the device before a new request and after another answer, the

device will ignore this answer but it will decrease the value of WD_CNT by the amount

WD_cnt_bad_step.

Seeds are 8-bit long words generated by a 7-bit LSFR pseudo-random algorithm. A new

seed is sent into SPI word (WD_Seed) each time a new seed request (T_start_REQ) is sent

to the device (if the last answer was correct).

GADG2401171548PS

WD_RST = OFF

Driver = OFF

WD_RST = ON

Driver = OFF

WD_RST = OFF)

Driver = ON

(Previous state was:

WD_RST = OFF

Driver = ON

WD_RST = OFF

Driver = ON

WD_RST = OFF)

Driver = OFF

(Previous state was:

WD_RST = OFF

Driver = OFF

WD_th_low

WD_th_high

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