L9779WD datasheet(37/170 Pages) STMICROELECTRONICS

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L9779WD

Functional description

169

6.4.2

VDD5_UV detection modes

VDD5_UV on RST unmasked without enabling VDD5_UV on MSC-SDO

Mode 1 is the default mode. A VDD5_UV event crates a reset of the whole system which

has the advantage that no special undervoltage topics concerning system behavior has to

be cared about. Disadvantage concerns requirements for being functional down to low Ubat.

Masking VDD5_UV on RST without enabling VDD5_UV on MSC-SDO

Advantage of this mode 2 is that the system remains fully functional even in a VDD5_UV

condition. This is especially interesting for systems whose requirements are to be functional

down to low Ubat. However it must be considered that also external components are still

functional at low Ubat. In mode 2 a VDD_UV condition is only detected by polling the

monitoring flag CRK_RST.

Masking VDD5_UV on RST and enabling VDD5_UV output on MSC-SDO

Advantage of this mode 3 compared to mode 2 is that a VDD5_UV event is detected fast as

no software polling of MSC register flag is necessary. This might be useful for external

functions who must be reset fast in case of a VDD_UV event. Advantage compared to

mode 1 is that the µC is not reset and therefore VDD5_UV recovery can be performed

faster.

If a VDD5_UV event occurs, MSC-SDO will go to low level. SDO will keep low permanently

even if VDD5 recovers. So it is guaranteed that even short VDD5_UV events are not

missed. The VDD5_UV condition is detected due to a MSC-SDO low pulse longer than the

length of an upstream frame. Upon detection the µC will have to go to its VDD5_UV

handling routine. There the µC will at first have to disable output of VDD5_UV on SDO for

re-enabling MSC communication and then start polling the CRK_RST flag to check if

undervoltage condition has healed or not. When undervoltage condition has healed and

CRK_RST flag is back to normal, recovery process can continue and the output of

VDD5_UV on MSC-SDO is enabled again for fast detection of an eventual next VDD5_UV

condition.

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