EM4022V20WS11 datasheet(10/15 Pages) EMMICRO

10 / 15 page

 2002, EM Microelectronic-Marin SA

www.emmicroelectronic.com

EM4022

Free running example

Transponder 1

Transponder 2

Transponder 3

Data stream in collision

Data stream detected

Fig. 10

Bi-directional protocols

Allowing bi-directional communication between reader

and transponders can speed up the basic free-running

protocol.

Communication

from

the

reader

transponders is achieved by turning the illuminating

energy field off for short periods.

The transponders

detect these gaps in the energy transmission and

interpret them as required.

Switch-off and Slow-down Modes

Reducing the effective population of transmitting

transponders in the reader field can speed up the free-

running protocol. One method to achieve this is by either

switching transponders off or slowing them down once

they have been detected. To achieve this, the reader

sends an ACK signal to a transponder after its code has

been successfully received. The transponder then either

switches off completely or reduces its repeat rate until it

is powered down. This reduces the number of collisions

between transponder transmissions, thereby reducing

the time required to read a group of tags.

Figure 11 shows a typical situation where a collision

occurs between transponder 1 and 3. Then, as soon as

transponder 2 is read, the reader sends an ACK signal to

this one switching it off as long hi is powered up from the

field. This eliminates the collision between transponder 1

and 2 in the next step The Switch-off protocol’s main

advantage is that identical transponders can be counted.

In the EM4022 the ACK signal is implemented as two

consecutive gaps with the appropriate timing and

received at a specific time after a code has been

transmitted.

The slow-down mode is a compromise between the free-

running mode and the switch-off mode. Etch time a

transponder is read, the reader send an ACK to double

the random repetition rate. This reduces the collisions

and in the time increases the saturation level.

Figure 12 show a typical case of this mode of operation.

Fast Mode

A second method of speeding up the reading of tags, is

to inhibit other transponders from transmitting while one

transponder is transmitting. This is done by sending a

MUTE signal to all of the transponders when the start of

a transmission is detected. The transponders stay muted

long enough (128 bit of its own clock) to allow the

transmission of one code (see figure 13). This allows the

transponder that has started transmitting to complete its

transmission without any collisions.

The other

transponders

continue

with

their

own

protocols

automatically after a time out, or continue immediately

upon detection of an ACK signal indicating that the

transmission

that

caused

the

MUTE

has

been

completed.

In the EM4022 the MUTE signal is implemented as a

single gap received while the transponder is not

transmitting.

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