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MCRF450

13.56 MHz Read/Write Passive RFID Device

Microchip Technology 

MCRF450/451/452/455

1.0 DESCRIPTION OF DEVICE

FEATURES

The MCRF450/451/452/455 is a contactless read/write

passive RFID device that is optimized for 13.56 MHz

RF carrier signal. The device needs an external LC

resonant circuit to communicate wirelessly with the

Interrogator. The device is powered remotely by

rectifying an RF signal that is transmitted from the

Interrogator and transmits or updates its contents from

memory-based on commands from the Interrogator.

The device is engineered to be used effectively for item

level tagging applications, such as retail and inventory

management, where a large volume of tags are read

and written in the same Interrogator field.

The device contains 32 blocks (B0-B31) of EEPROM

memory. Each block consists of 32 bits. The first three

blocks (B0-B2) are allocated for device operation, while

the remaining 29 blocks (B3-B31: 928 bits) are for user

data. Block 1 contains unique 32 bits of Tag ID. The Tag

ID is preprogrammed at the factory and write protected.

All blocks, except for the Tag ID (Block 1), are contact-

lessly writable block-wise by Interrogator commands.

All data blocks, with the exception of bits 30 and 31 in

Block 0, are write-protectable.

The device can be configured as either Tag-Talks-First

(TTF) or Interrogator-Talks-First (ITF). In TTF mode,

the device transmits its fast response data (160 bits

max., see Example 9-1) as soon as it is energized, then

waits for the next command. In ITF mode, the device

requires an Interrogator command before it sends any

data. The control bits for TTF and ITF modes are bits

30 and 31 in Block 0.

All downlink commands from the Interrogator are

encoded using 1-of-16 Pulse Position Modulation

(PPM) and specially timed gap pulses. This encoded

information amplitude modulates the Interrogator’s RF

carrier signal.

At the other end, the MCRF450/451/452/455 device

demodulates the received RF signal and then sends

data (from memory) at 70 Kbit/s back to the Interrogator

in Manchester format.

The communication between Interrogator and device

takes place asynchronously. Therefore, to enhance the

detection accuracy of the device, the Interrogator

sends a time reference signal (time calibration pulse) to

the device, followed by the command and program-

ming data. The time reference signal is used to

calibrate timing of the internal decoder of the device.

There are device options for the internal resonant

capacitor between antenna A and VSS: (a) no internal

resonant capacitor for the MCRF450, (b) 100 pF for the

MCRF451, (c) two 50 pF in series (25 pF in total) for

the MCRF452 and (d) 50 pF for the MCRF455. The

internal resonant capacitors for each device are shown

in Figures 2-2 through 2-5.

The MCRF450 needs an external LC resonant circuit

connected between antenna A, antenna B and VSS

pads. See Figure 2-2 for the external circuit configura-

tion. The MCRF452 needs a single external antenna

coil only between antenna A and VSS pads, as shown

in Figure 2-4.

This external circuit, along with the internal resonant

capacitor, must be tuned to the carrier frequency of the

Interrogator for maximum performance.

When a tag (device with the external LC resonant

circuit) is brought to the Interrogator’s RF field, it

develops an RF voltage across the external circuit. The

device rectifies the RF voltage and develops a DC

voltage (VDD). The device becomes functional as soon

as VDD reaches the operating voltage level.

The device then sends data stored in memory to the

Interrogator by turning on/off the internal modulation

transistor. This internal modulation transistor is located

between antenna B and VSS. The modulation transistor

has a very small turn-on resistance between Drain

(antenna B) and Source (VSS) terminals during its turn-on

time.

When the modulation transistor turns on, the resonant

circuit component between antenna B and VSS, which

is in parallel with the modulation transistor, is shorted

due to the low turn-on resistance. This results in a

change in the LC value of the circuit. As a result, the

circuit no longer resonates at the carrier frequency of

the Interrogator. Therefore, the voltage across the

circuit is minimized. This condition is called “cloaking”.

When the modulation transistor turns off, the circuit

resonates at the carrier frequency of the Interrogator

and develops maximum voltage. This condition is

called “uncloaking”. Therefore, the data is sent to the

Interrogator by turning on (cloaking) and off

(uncloaking) the modulation transistor.

The voltage amplitude of the carrier signal across the

LC resonant circuit changes depending on the

amplitude of modulation data. This is called an ampli-

tude modulation signal. The receiver channel in the

Interrogator detects this amplitude modulation signal

and reconstructs the modulation data for decoding.

The device includes a unique anti-collision algorithm to

be read or written effectively in multiple tag environ-

ments. To minimize data collision, the algorithm utilizes

time division multiplexing of the device response. Each

device can communicate with the Interrogator in a

different time slot. The devices in the Interrogator’s RF

field remain in a nonmodulating condition if they are not

in the given time slot. This enables the Interrogator to

communicate with the multiple devices one at a time

without data collision. The details of the algorithm are

described in Section 6.0 “Read/Write Anti-Collision

Logic”.

 2003 Microchip Technology Inc.

DS40232H-page 3

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