mc33789 Freescale Semiconductor, Inc, mc33789 Datasheet - Page 68

• Manufacturer Part Number: mc33789
• Description: Airbag System Basis Chip Sbc With Power Supply And Psi5 Sensor Interface
• Manufacturer: Freescale Semiconductor, Inc

FUNCTIONAL DEVICE OPERATION
SAFING
interface. It is not possible to set both of the outputs in their
active states (ARM=1 and DISARM=0) in this mode.
command is received for either a Safing mode transition or a
Scrap mode transition.
Safing Mode
field set to 'Safing' while in Diagnostic mode, the SSM enters
Safing mode.
operation. The safing logic can perform all of the safing
functions, including monitoring sensor data and setting the
ARM/DISARM signals for Arming output while in this state.
the SSM Reset signal.
outputs is illustrated in
seed value;
68
33789
Valid Key :
The SSM remains in this mode until a SAFE_CTL SPI
Upon reception of the SAFE_CTL command with the CTL
Safing mode is the primary run-time mode for normal
The only means of exiting Safing mode is the assertion of
The handshake sequence for activating the Arming
The 33789 uses a free-running 8-bit counter to generate a
determined by Safing)
KEY received before scrap
timeout is expired
KEY value correct
(ARM, DISARM
The MCU read the seed value with the
SCRAP_SEED command;
AND
The MCU uses the seed value to generate an 8-
bit key;
The MCU submits the key value with the
SCRAP_KEY command;
SSM Reset
Figures
SAFE_CTL = Safing
WSM = DRIVE
Safing Mode
52. The strategy is:
Start-up Mode
Correct ACL
Valid key
AND
Figure 51. Safing State Diagram
Incorrect ACL
Key timeout OR
Invalid Key OR
(ARM = 0, DISARM = 1)
Arming Mode
Diag Mode
WSM = DRIVE
OR
[WSM = WDOG OVERRIDE and SPI WDOG Feed]
Scrap Mode
the CTL field set to 'Scrap', while the SSM is in Diagnostic
mode, it will force the SSM enter into Scrap mode.
the sensor interface, and the safing compare logic is
disabled.
mode only if the MCU initiates the transition. The only way to
move it back from Scrap mode to Start-up mode is through an
SSM Reset.
Arming Mode
(ARM = 1 and DISARM = 0) to enable firing of squibs.
mode, and to prevent undesired activation of the safing
signal, a handshake is used to control entering into and
exiting out of Arming mode.
If the MCU sends the 33789 a SAFE_CTL command with
Once entered into Scrap mode, the SSM stops monitoring
From Scrap mode, the SSM can be moved into Arming
While in Arming Mode, the safing outputs are asserted
In order to protect from an inadvertent enter into Arming
(ARM, DISARM can be tested:
ARM = 0 and DISARM = 0, or
ARM = 1 and DISARM = 1)
Transition into Arming mode
(ARM = 1, DISARM = 0)
The 33789 freezes the seed value read by the MCU
and computes its own key;
Scrap Mode
SAFE_CTL = Scrap
WSM = DRIVE
Valid Key
AND
Correct ACL
Compares two key values at the 33789;
The 33789 receives the ACL signal at the
SCRAP pin from the MCU;
Analog Integrated Circuit Device Data
If two key values match each other,
the ACL signal is valid,
AND
Incorrect ACL
First Scrap KEY
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