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Analog Front End IC TMS37122 - Reference Guide
August ’01
3.4
Immobilizer Function
The current Immobilizer function could be desired additional (backup) or instead of Passive
Start function. In this case the 3D-AFE together with a Microcomputer could even replace a
TIRIS Transponder because compatibility can be achieved.
The car driver has to place in this case the Identification Device near a car-internal LF Trans-
ceiver Antenna. This can for example be the traditional Keylock Reader Antenna or another
reader in the dashboard.
The car driver is triggering the immobilization process using a push button or a mechanical
key. This causes the Transceiver to send out a charge burst (typical 50 ms). The field strength
and coupling between Transceiver and Identification Device should be high enough that the
charge capacitor is charged to maximum.
For full compatibility it is assumed that the Wake Pattern detection of 3D-AFE is disabled.
Due to this the 3D-AFE enters at once the Transparent Mode and the Microcomputer inter-
prets the Commands and Data eventually sent after the charge. If enough supply voltage is
available because of intact battery or charge voltage (battery backup), a Response can be pre-
pared and sent back using the procedure described in previous section (see Figure 31).
3.5
Anti-Collision Function
If more than one Identification Device is in the field during a Passive Entry or Passive Start
function, the simultaneous UHF Responses initiated by a Challenge, might interfere each oth-
er. A solution to this problem is described in a TIRIS Patent [2], which is allowed to use in
conjunction with the TMS37122.
The Identification Device Microcomputer must have stored for this purpose in its memory a
'Key Number' and eventually a 'Car Number'. The Wake Pattern also fulfils the task of a 'Car
Number'. Let us suppose that the system is designed for maximum four Identification Devices
per car. These four devices have the same 'Car Number' but different 'Key Numbers' (for ex-
ample: 1, 2, 3,4).
Interference by foreign Identification Devices can be easily avoided by sending the 'Car
Number' (Wake Pattern) together with Command and Challenge. The Identification Device
will only respond if the 'Car Number' (Wake Pattern) matches.
To avoid collision of the four valid Identification Devices the following procedure must be
implemented in the software:
Whenever the 3D-AFE is initialized (WDEEN = high) a software counter is set to zero. After
receipt and Encryption of a Challenge the next EOBA signal is awaited. Instead of transmit-
ting at once the Response the software increment with this signal the counter and compares
the content with the 'Key Number' in the memory.
Only if equal the Response is performed. Otherwise next EOBA is awaited. So the four de-
vices will respond in separate time windows (see Figure 32).
After the counter has reached the maximum key number, the 3D-AFE is re-initialized.
If the Identification Device does not manage to count to the maximum number within the du-
ration of three responses, the circuit is also reset.
