L9680 datasheet(210/277 Pages) STMICROELECTRONICS

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Safing logic

L9680

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DocID029257 Rev 1

Figure 62. 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 63. The

strategy involves generation of a seed value from within the L9680 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), L9680 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 L9680 transitions from Scrap state to

Arming state. To remain in Arming state the microcontroller must periodically refresh L9680

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

command, and L9680 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.

L9680 Datasheet(PDF) 210 Page - STMicroelectronics