L9300 datasheet(79/182 Pages) STMICROELECTRONICS

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When PRN signal, on PRN pin, is missing for a timeout duration (Toff_to or Toff), a low level signal will be present

on RES1 pin and a Time_out diagnostic flag is latched on Service Fault SPI register. After TRL period of time,

high level signal will be present again on RES1 pin. If PRN signal continues to be missing, even after the RES1

signal returns to high, or in case a discontinuous PRN signal is provided on PRN pin, the RES1 pin is repetitively

driven to low and high the above operation of turning low/high repeats frequently. RES1 generation can be

masked through SPI bit in the Service Enable register to let MCU test the watchdog functionality: microcontroller

may stop serving PRN pin causing watchdog fault generation without being reset in order to check the PRN

functionality. At power-up of the device, after RES1 has been released, the watchdog is initialized. At start-up, in

order to allow microcontroller to finish its boot sequence and starting to serve PRN pin continuously, the watchdog

waits for a valid PRN pulse (Figure 47) until a time out time Toff_to; as soon as a valid PRN pulse is provided

the Toff_to timing is stopped and the watchdog timer switches to nominal Toff timings. Toff_to is triggered at each

RES1 rising edge, Toff is triggered at each PRN rising edge.

Figure 45. WD time-out timings at power up

GADG0408170852PS

PRN_to Counter

RES1

Toff_to

Toff

TRL

PRUN

PRN Counter

PRN Watchdog can be enabled or disabled through the WD_DIS pin, internally pulled down.

In case disable mode is selected (WD_DIS = HIGH), a RES1 won’t be generated even if PRN signal, on PRN pin,

is missing.

If WD_DIS pin is left open or it is pulled down to ground, PRN Watchdog is enabled.

In case disable mode is selected (WD_DIS = high), the above operation never appears even in case of no PRN

signal.

An echo of WD_DIS signal is present into SPI registers to validate pin status from external.

In case no PRN signal is present at WD_DIS falling edge the watchdog starts with timeout counter as in case of

RES1 release; timeout is stopped at first PRN rising edge signal.

L9300

PRUN watchdog functional description

DS12258 - Rev 4

page 79/182

L9300 Datasheet(PDF) 79 Page - STMicroelectronics