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MCRF450

13.56 MHz Read/Write Passive RFID Device

Microchip Technology 

MCRF450/451/452/455

6.2.3

DETECTION OF INTERROGATOR

COMMANDS

The Interrogator sends commands to the device by

amplitude modulating the carrier signal (gap pulse).

The Interrogator uses two classes of encoding signals

for modulation. They are (1) 1-of-16 PPM for data

transmission, and (2) specially timed gap pulse

sequence for the FRR and FRB commands. These

commands consist of five gap pulses within nine possi-

ble gap pulse positions (1.575 ms). The combination of

the possible gap positions determines the command

type and parameters of the Fast Read command.

The Interrogator also sends TCP prior to the 1-of-

16 PPM. The TCP is used to calibrate the time-base of

the decoder in the device. The specifics of the two

encoding methods and the TCP are described in the

following sections.

6.2.3.1 Fast Read (FR) Commands

The FR commands are composed of five 175 µs wide

gap pulses (see Figure 6-2) whose spacing within

1.575 ms determines the command type and its param-

eters. Table 6-4 shows the specification of the gap

signal for the FR commands. Two commands are used

for the fast read. They are: (1) Fast Read Request

(FRR) in the Detection loop, and (2) Fast Read Bypass

(FRB) in the Reactivation loop. See Tables 6-5 and 6-6

for the FRR gap pulse positions. See Figures 6-3 to 6-8

for the gap modulation patterns.

The parameters of FRR are: (1) number of time slots

(TSMAX = 1,16, or 64), (2) maximum transmission

counter (TCMAX) and (3) data transmission speed.

The FRB has only a data transmission speed parame-

ter (Normal or Fast Speed mode). The device extracts

these parameters based on the positions of the five gap

pulses within the 1.575 ms time span, as shown in

Figures 6-3 to 6-8.

TSMAX = 1 is given if there is only one device in the

field. This is called “Conveyor mode” or “single tag

environment”. In this mode, the device responds with

the FR response signal in every time slot until it

receives a correct matching code, or until TCMAX is

elapsed.

6.2.3.2 Data Transmission Speed

The Interrogator can send data with two different data

rates: (1) Normal and (2) Fast Speed modes. The

normal speed uses 2.8 ms/symbol, while the fast

speed uses 160 µs/symbol. One symbol represents

one 4-bit data packet (see Section 6.2.3.4 “1-of-16

PPM”). The data transmission speed is a parameter of

the FRR and FRB commands. This parameter

indicates the data speed of subsequent Interrogator

commands. The data rate of the device output (70 kHz)

is not affected by this parameter.

TABLE 6-4: SPECIFICATION OF GAP SIGNAL FOR FRR AND FRB COMMANDS

Number of gaps for one command

5

Total available number of gap positions within the command time span

9

Command time span

1.575 ms

Gap pulse width

175 µs

DS40232H-page 22

 2003 Microchip Technology Inc.

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