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4688D–RFID–03/07
U3280M
3. Functional Description
3.1 Transponder Interface
The U3280M is a transponder interface IC that can operate microcontrollers using wireless tech-
nology and battery independently. Wireless data communication and the power supply are
handled via an electromagnetic field and the coil antenna of the transponder interface. The
U3280M consists of a rectifier stage for the antenna, power management to handle field and
battery power supplies, a damping modulator, and a field-gap detection stage for contactless
data communication. Furthermore, a field clock extraction and an EEPROM are on-chip.
The internal rectifier stage rectifies the AC from the LC-resonant circuit at the coil inputs and
supplies the U3280M device and an additional microcontroller device with power. It is also possi-
ble to supply the device via the V
Batt
input with DC from a battery. The power management
handles switching between battery supply (V
Batt
pin) and field supply automatically. It switches to
field supply if a field is applied at the coil, and it switches back to battery if the field is removed.
The voltage from the coil or the V
Batt
pin is output at the V
DD
pin to supply the microcontroller or
any other suited device. At the V
DD
pin a capacitor must be connected to smooth and buffer the
supply voltage. This capacitor is also necessary to buffer the supply voltage during communica-
tion (damping and gaps in the field).
For communication, the chip contains a damping stage and gap-detect circuitry. By means of the
damping stage the coil voltage can be modulated to transmit data via the field. It can be con-
trolled with the modulator input (MOD pin) via the microcontroller. The gap-detection circuitry
detects gaps in the field and outputs the gap/field signal at the gap-detect output (Pin NGAP).
To store data like keycodes, identifiers and configuration bits, a 512-bit EEPROM is available
on-chip. It can be read and written by the microcontroller via a two-wire serial interface.
The serial interface, the EEPROM and the microcontroller are supplied with the voltage at the
V
DD
pin. That means the microcontroller can read and write the EEPROM if the supply voltage at
V
DD
is in the operating range of the IC.
The U3280M has built-in operating modes to support a wide range of applications. These modes
can be activated via the serial interface with special mode control bytes.
To support applications with battery supply only, power management can be switched off by
software to disable the automatic switching to field supply.
An on-chip Bi-phase and Manchester modulator can be activated and controlled by the serial
interface. If this modulator is used, it modulates the serial data stream at the serial inputs SDA
and SCL into a Bi-phase or Manchester-coded signal for the damping stage.
3.2 Modulation
The transponder interface can modulate the magnetic field by its damping stage to transmit data
to a base station. It modulates the coil voltage by varying the coil’s load. The modulator can be
controlled via the MOD pin. A high level (“1”) increases the current into the coil and damps the
coil voltage. A low level (“0”) decreases the current and increases the coil voltage. The modula-
tor generates a voltage stroke of about 2 V
pp
at the coil. A high level at the MOD pin makes the
maximum of the field energy available at V
DD
. During reset mode, a high level at the MOD pin
causes optimum conditions for starting the device and charging the capacitor at V
DD
after the
field has been applied at the coil.
