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MLX90111

128bit Read Write Transponder

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MLX90111

128bit Read Write Transponder

General Description

The MLX90111 is a 128bit Read Write

transponder IC.

The integrated tune

capacitance and the external coil form a

resonant LC antenna that absorbs part of the

electromagnetic energy radiated by the

transceiver LC antenna. Physically, a magnetic

coupling occurs between the transceiver, also

referred to as "reader", and transponder

antennas

To amplitude modulate (AM) the RF carrier, the

transponder damps the electromagnetic field by

switching a resistor in parallel with its coil. This

way the transponder repetitively transmits its

128bit-identification code (ID) to the transceiver,

which recovers the envelope of the damped

field, and decodes the ID.

The transceiver can send commands and data

to the transponder by modulating the amplitude

of the carrier.

POR level

When the transponder is placed in an external

radio frequency (RF) field of appropriate

frequency and amplitude, the internal power

supply (VDD) can build up as charge on the

integrated buffer capacitance. The modulation

resistor is switched on and off as soon as VDD

has reached the Power On Reset (POR) level.

When the modulation resistor is switched on, the

LC circuit is no longer tuned, therefore no

energy is taken from the field, and VDD drops

due to internal consumption. Hence, by

switching the modulation resistor on and off, it is

possible to oscillate around the POR level. This

phenomenon is referred to as hiccuping.

The modulation resistor is switched off when the

transponder drops below the POR level.

Sensitivity level

The minimum electromagnetic force (e.m.f.)

needed to avoid hiccuping is defined by

Vacsens. At this level the power supply will not

drop below the POR level while the modulation

resistance is switched on during modulation.

Clock recovery

The transponder takes its clock from the carrier

frequency. Depending on the speed option, the

carrier frequency is divided by 32(4kbaud),

64(2kbaud) to generate the bit rate.

Demodulator

If the external RF field is AM modulated

according to the asynchronous control pattern

(ACP), the transponder stops transmitting the ID

and enters the Program Mode.

Data is then sent to the transponder using

Return to One encoding. First half bit is low (0)

or high (1), and the second half is high. In the

middle of each half bit one sample is taken. In

order for the transponder to detect the ACP

modulation a minimum modulation depth is

required. See timing specification below for

more details on the write protocol.

RF limiter, or clamping

The build up of voltage on the resonant LC

circuit is limited to avoid damaging the internal

circuit.

Modulator

The modulator consists of a modulation resistor

that is switched in parallel with the resonant LC

circuit (antenna). Because in weak RF fields the

rising edge is much slower than the falling edge

of the envelope, a symmetrically driven

modulator would give an asymmetrical envelope.

This is anticipated by delaying each falling edge

by a fixed number of RF clock pulses. The

modulator is hence driven asymmetrical. Each

ON state is reduced by 8 (4) clocks in 2 (4)

kBaud mode, and each OFF state is prolonged

by the same amount.

Speed Total clocks

per Bit

2kbaud

64

4kbaud

32

Clocks ‘ON’ state

Long ON = 64 - 8

Short ON = 32 - 8

Long ON = 32 – 4

Short ON = 16 – 4

Clocks ‘OFF’ state

Long ON = 64 + 8

Short ON = 32 + 8

Long ON = 32 + 4

Short ON = 16 + 4

MLX90111 128bit Read Write Transponder Page 5 of 12

Rev 1.12 5-Feb-01

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