L9679P datasheet(204/272 Pages) STMICROELECTRONICS

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Safing logic

L9679P

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DocID029275 Rev 1

Figure 61. Pulse stretch timer example

The Arming Enable Pulse Stretch Timer status is available in the AEPSTS register.

12.6

Additional communication line

The ACL pin is the Additional Communication Line input that provides a means of safely

activating the arming outputs (ARMx and VSF) for disposal of restraints devices at the end

of vehicle life.

The handshake sequence for activating the Arming outputs is illustrated in Figure 62. The

strategy involves generation of a seed value from within the L9679P device using a free-

running 8-bit counter running at fSCRAP_SEED rate, where it can be read by the

microcontroller. The microcontroller uses it to generate an 8-bit key value. When the seed

value is read (SPI SCRAP_SEED command), L9679P also freezes the seed value and

computes its own key, which is used for comparison to the key subsequently submitted by

the microcontroller. The key value is submitted by the microcontroller using the

SCRAP_KEY command, and successful reception of this command with a key value

matching the internally calculated key allows the successful completion of the first

handshake. After that, in case a second handshake (seed-key) completes successfully and

if a valid ACL is detected (as described below) the L9679P transitions from Scrap state to

Arming state. To remain in Arming state the microcontroller must periodically refresh

L9679P with the SCRAP_KEY command containing the correct key value in the data field of

the command, and L9679P must also receive the correct ACL signal. This must occur

before the scrap timeout timer expires (TSCRAP_TIMEOUT). The scrap key is derived from the

seed value using a simple logical inversion on the even-numbered bits (0, 2, 4, 6). From a

logical standpoint, this is equivalent to a bit-wise XOR of the seed value with 0x55.

While the SSM is in Arming state, the arming outputs are asserted (ARMx=1, VSF on). If the

periodic scrap key is incorrect, or not received before the timeout expires, or the ACL is not

correctly received, the SSM reverts back to the Scrap state, and the arming outputs are

deactivated.

L9679P 数据表(PDF) 204 Page - STMicroelectronics