L9779WD datasheet(125/170 Pages) STMICROELECTRONICS

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L9779WD

Functional description

169

6.17.5

MicroSecond Channel activity watchdog

MSC data frames are monitored to be sent in intervals shorter than tMSC_mon. If L9779WD

receives no valid data frame for longer than tMSC_mon, it will switch off all the drivers and

the error flag (TRANS_F) and OUT_DIS will be set.

The MRD and OUT13, 14, 21 and 25 (if low battery function is enabled) are not disabled by

missing activity on MSC.

No reset request is sent to the smart reset function module.

To enable the outputs again, the µC has to read the TRANS_F and then send the command

START, and then outputs are reactivated with the first correct data frame. If the fault flag is

not cleared the START command is ignored.

By default the MicroSecond Channel activity watch dog is enabled and the monitoring time

will start after writing of the OUT_DIS bit by START command. Each time the L9779WD

receives a valid data frame the tMSC_on timer is reset. This means that micro controller can

drive the outputs only when the monitoring module is active.

To disable the MicroSecond Channel activity watch dog the µC have to set to 0 the bit

MSC_ACT_EN.

If the MSC frame has a wrong number of bit the flag TRANS_L is set but no action on

outputs is taken. The frame with wrong length is ignored.

Table 58. MicroSecond Channel activity watchdog

Symbol

Min

Typ

Max

Unit

tMSC_mon

100

142

185

µs

t1WD

-30%

0.9*t2WD

+30%

-30%

0.8*t2WD

+30%

-30%

0.7*t2WD

+30%

-0-

t2WD

100

130

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