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Analog Front End IC TMS37122 - Reference Guide August ’01
3.2 Passive Start Function
A further application of 3D-AFE is the Passive Start function. The car driver is already seated
in the car and triggers the Base Station by pushing a button at the dashboard or at the middle
console. The Base Station transmits now the Passive Start (PS) Wake Pattern, a Command
and a Challenge using an indoor Antenna, located for example in the dashboard, seat or door.
The Antenna is driven by a LF Transceiver, which is typically activated with an active low
signal called TXCT- (see Figure 30).
It is assumed that the Identification Device is in Standby Mode (WDEEN=high). During
Wake-up Time the IC internal wake (WAKEI) will be activated if the threshold VWAKEA
is exceeded at one or more antennas. The first WAKEI signal determines the receive channel.
During the Waiting Time (twait) the Base Station will start modulation while sending the PS
Wake Pattern. The internal clock CLKI will be active during this period.
If the received PS Wake Pattern is equal to the PS Wake Pattern in the EEPROM, the RF level
check with VWAKEB will be performed (sensitivity stays at VWAKEA level). If the level is
higher than VWAKEB, the output WAKE is set. This will cause the Microcomputer of the
Identification Device to wake-up after an eventually µC Wake Delay. Depending on the re-
quired time, the Base Station should insert an active period between Wake Pattern and Com-
mand transmission.
The digitized amplitude modulation is output at the EOBA and demodulation is done by the
Microcomputer. For this purpose the CLKA signal can be used as reference, which is avail-
able at output EOBA/M.
The Command sent to the Microcomputer must include an information that a Passive Start
action is requested.
After checking the Command, the Microcomputer knows that a Passive Start function is re-
quested. The Challenge will be encrypted (ENCR), respectively the Rolling Code will be de-
termined depending on the system. Then the Microcomputer waits for the next EOBA signal
created by the Base Station, switching off the Transmitter. When this has happened, the UHF
Transmitter is activated and the Response Telegram is sent to the Base Station. There the Re-
sponse is checked and the engine is started if valid.
The Microcomputer initializes the 3D-AFE after transmission of the Response with a short
negative pulse at WDEEN. The circuit returns to Standby Mode and is ready for next cycle.
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