L9679P datasheet(37/272 Pages) STMICROELECTRONICS

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L9679P

Start-up and power control

271

6.2.6

IC operating states

Different states can be identified while operating the device. These states allow safe and

predictable initialization, test, operation and final disposal of the part (scrapping).

As soon as the RESET signal is de-asserted at the beginning of the ACTIVE mode, the

microcontroller powers up. At this stage, L9679P is in the Init state: during this state the

device must be initialized by the controller. In particular, the watchdog timer window can be

programmed during this state.

When the watchdog service begins (upon the first successful watchdog feed), the device

switches to Diag state for diagnostics purposes. The remaining configuration of the device is

allowed in this state, in particular for safing records and deployment masks. Several tests

are also enabled while in this state and all these tests are mutually exclusive to one another.

HS and LS switch tests of the squib drivers can only be processed during this Diag state.

Also high side safing FET can only be run during this state. When not in Diag state, any

commands for squib driver switch tests will be ignored. Other checks are also performed: on

the arming outputs to check for non-stuck-at conditions on the pins and on the configured

firing time configuration through one of the ARMx pin. The SSM remains in this state until

commanded to transition into the Safing state or Scrap state via the dedicated SPI

commands.

Upon reception of the SAFING_STATE command while in Diag state, the device enters

Safing state. This is the primary run-time state for normal operation, and the logic performs

the safing function, including monitoring of sensor data and setting of the ARMx signals. The

only means of exiting Safing state is by the assertion of the SSM_Reset signal.

The Scrap state is entered upon reception of the SCRAP_STATE command while in Diag

state. While in Scrap state, the part allows the main microcontroller to initiate a transition to

Arming state, and monitoring of the Remote Sensor SPI interface and the safing logic is

disabled. From Scrap state, the device can transition to Arming state only, and the only

means of moving back to Init state is through an SSM_Reset.

In order to protect from inadvertent entry into Arming state, and to prevent undesired

activation of the safing signals, a handshake mechanism is used to control entry into, and

exit from Arming state. This handshake is described further in Section 11.6. While in Arming

state, the arming outputs are asserted. Exit from Arming state occurs when the periodic

SCRAP_KEY commands cease (timeout), the key value is incorrect, or when SSM_Reset is

asserted. Upon exit, the device re-enters Scrap state, except for the case of SSM_Reset,

which results in entry into Init state.

The device operating states are shown in Figure 10.

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