R

EM4223

Copyright

© 2005, EM Microelectronic-Marin SA

21

www.emmicroelectronic.com

8.

TRANSPONDER SELECTION OPERATION

– INIT_ROUND AND BEGIN_ROUND

COMMANDS

The INIT_ROUND and BEGIN_ROUND commands have

the ability to move only a selected sub-set of the

Transponder population from the READY to the

ROUND_ACTIVE states. Transponders that are already

in the ROUND_ACTIVE or ROUND_STANDBY states

will be removed from the active Transponder population

and moved to the READY state if they do not match the

selection parameters sent with the INIT_ROUND or

BEGIN_ROUND command.

This allows the population to be “thinned”, thus

increasing

the

effective

read

rate

achieved.

Because only transponders of interest to the application

will be selected any other Transponders in the Reader

field will not degrade Reader performance by needing to

be read and acknowledge to send them to the QUIET

state – they virtually do not exist if they have not been

selected.

The selection capabilities also allow the Transponder

population

to

be

“Tree-Walked”

allowing

fully

“Deterministic” arbitration of a Transponder population.

By adding more and more bits to the selection criteria,

the population can be resolved down to a single

Transponder. (See the explanatory note below).

EXPLANATION OF “DETERMINISTIC” OPERATION BASED ON “TREE-WALKING

”

Transponders that use randomly selected reply slots in order to transmit their data to a Reader have a

very small risk of more than one Transponder selecting the same slot several times, which could mean

that such tags may not be read before they move out of the active population. This is known as

“Probabalistic” operation and must be balanced against the many advantages of this mode of operation.

“Tree Walking” is a method of resolving Transponder populations by effectively issuing a series of “tests”

or “challenges” in which the Reader would request a response from all tags containing say “0” in the 1

position of the Transponder data (or in an encrypted version of the data). If the Reader received a non-

clashing response (only 1 transponder responding) it could request that Transponder to send its full data.

If the Reader received a clashing response (more than 1 transponder responding) it would know that it

had identified a productive “branch” and would extend its test by requesting a response from all tags

containing say “00” in the 1

requesting responses until it had resolved the entire tag population in this manner. If the Reader received

no response it would know that it had identified an unproductive “branch” and would temporarily abandon

further testing for Transponders with “0” in the 1

Transponders with “1” in the first bit position. This would continue until all Transponders had been

identified, or moved out of the Reader’s RF field.

INIT_ROUND COMMAND SELECTION OPERATION

(see

70H70H

Fig. 19)

The INIT_ROUND command contains a single fixed

length (8 bit) selection parameter. This parameter

represents

the

AFI

(Application

Family

Identifier

according to ISO18000-6A) value which will be matched

with the AFI value contained in the Transponders

memory. Transponders with a matching AFI value will

move from the ROUND_ACTIVE or ROUND_STANDBY

or READY states to the ROUND_ACTIVE state and

commence participation in the Arbitration process.

Transponders that do not match the AFI value sent in the

command will remain in the READY state or they will

move to the READY state if they are already in the

ROUND_ACTIVE or ROUND_STANDBY states.

If the AFI value contained in the INIT_ROUND command

is 0x00, the Transponders will ignore the parameter in

the command and all Transponders will move to the

ROUND_ACTIVE state from the ROUND_ACTIVE or

ROUND_STANDBY or READY states. With an AFI

parameter of 0x00, the command will perform identically

to an INIT_ROUND_ALL command.

Tags in the QUIET state will ignore the INIT_ROUND

command.