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Analog Front End IC TMS37122 - Reference Guide
August ’01
3.3
Battery Backup Function
It is assumed that the system is configured to work with Battery Backup Supply (see Figure
7) that means VCCO is able to supply VBAT. Battery voltage is so low that the Supervisory
Circuit has reset the Microcomputer (see Figure 9). Due to this WDEEN is low and the whole
Identification Device consumes a low quiescent current.
The car driver tries to get into the car by pulling the door handle while the Identification De-
vice is still in far distance range of the Base Station Antenna. The Base Station sends out the
Passive Entry Wake Pattern, Passive Entry Command and the Challenge, but gets no Re-
sponse, because the Identification Device is off.
The Base Station controller in such a case will repeat the Challenge several times. Because it
takes into account that the battery of the Identification Device is empty, it increases the Wake-
up Time and sends out the PE Backup Command.
The car driver, who recognizes that the door does not open, brings the Identification Device
nearer to the back mirror, because he was advised to do by the car’s user manual. As a result
the charge capacitor in the Identification Device is charged up to a higher voltage (see Figure
31). Via the voltage regulator in the 3D-AFE, the supply voltage of Microcomputer and Front
End is increased. The Supervisory Circuit will detect sufficient voltage and will restart the
Microcomputer. After initialization the Microcomputer will enable the 3D-AFE and in trans-
parent mode the Backup Command and the Challenge will be received.
The Microcomputer now knows that the Response cannot be sent via UHF Channel, because
current consumption will be too high. Therefore it uses the LF Transmission Mode. By setting
TX to high with occurrence of last EOBA signal, an oscillation is started at Resonant Circuit
1. The Response is transmitted via LF channel using FSK controlled by MOD input. The data
to be transmitted are changed synchronous to the CLKM signal available at output CLKA/M.
Timing requirements and data formats of the TIRIS System and Transponders should be ob-
served [1].
The LF data are received by the Transceiver connected to the Mirror Antenna and checked
by the Base Station Controller.
After transmission of the whole Response the Microcomputer resets the TX input and initial-
izes the 3D-AFE using WDEEN.