8
Analog Front End IC TMS37122 - Reference Guide
August ’01
1.3
Circuit Description
The 3D-Analog Front End (AFE) circuit TMS37122 is a low voltage, low current interface
circuit for microcomputer controlled Automotive Keyless Entry Systems. With few external
components it provides a 120 to 140 kHz inductive Low Frequency Channel for high-speed
data transfer to a microcomputer or other control circuits.
Typically the circuit is used to expand current Remote Keyless Entry systems, consisting of
a Microcomputer and an UHF Transmitter, to Passive (Keyless) Entry function see Figure 2.
The battery-powered device is able to detect amplitude modulated Low Frequency (LF) sig-
nals from a LF Transmitter/Receiver Unit from an extended distance compared to battery-less
devices.
The modulation principle used by the Transmitter should be based on a 100% amplitude mod-
ulation. Pulse Position Modulation (negative transition) must be used for transmission of an
optional wake pattern, which is demodulated in the circuit without Microcomputer involve-
ment. The encoding scheme of the remaining protocol data is free selectable depending on
system requirements, hardware and software capability.
The achievable distance between transmitter and transponder and the possible data speed de-
pends on the size and quality factor of the antennas. Typical achievable distances are in the
range of 1.5 to 2.5 m.
Figure 2: System Arrangement of 3D-Analog Front End IC TMS37122
The 3D-AFE can be equipped with up to three antennas covering the three space axes to avoid
reduced detection ranges due to antenna orientation (see Figure 3). The antenna resonant cir-
cuits can be tuned independently to resonance with internal EEPROM-controlled trim capac-
itor arrays accessible by a digital Trim and Test Interface.
The circuit can be completely disabled by the Microcomputer and consumes in this state a
very low quiescent current (I
quiet
). In stand-by mode the current consumption is still so low
that a continuous application is feasible (I
stdb
). Even in case of interference the circuit has a
low current consumption due to watchdog circuit and supervision logic. The Microcomputer
is only woken and supplied with the recovered digital data and clock when a valid signal is
received.
Antenna 1 LF Res-
ponse for Backup
LF
TX/RX
Unit
(ASIC)
UHF
Receiver
Control
Unit
LF Write-Channel
UHF
Response-Channel
UHF
Tx/Osc
3D
AFE
Identification Device
Transceiver / Base Station
RKE
buttons
Driver
Trigger
(Door Handle)
uC
(customer
design)
