ST25TB02K datasheet(18/47 Pages) STMICROELECTRONICS

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Anticollision

The ST25TB02K provides an anticollision mechanism that searches for the Chip_ID of each device that is present

in the reader field range. When known, the Chip_ID is used to select an ST25TB02K individually, and access

its memory. The anticollision sequence is managed by the reader through a set of commands described in

Section 8 ST25TB02K commands:

•

Initiate()

•

Pcall16()

•

Slot_marker().

The reader is the master of the communication with one or more ST25TB02K device(s). It initiates the tag

communication activity by issuing an Initiate(), Pcall16() or Slot_marker() command to prompt the ST25TB02K

to answer. During the anticollision sequence, it might happen that two or more ST25TB02K devices respond

simultaneously, so causing a collision. The command set allows the reader to handle the sequence, to separate

ST25TB02K transmissions into different time slots. Once the anticollision sequence has completed, ST25TB02K

communication is fully under the control of the reader, allowing only one ST25TB02K to transmit at a time.

The Anticollision scheme is based on the definition of time slots during which the ST25TB02K devices are invited

to answer with minimum identification data: the Chip_ID. The number of slots is fixed at 16 for the Pcall16()

command. For the Initiate() command, there is no slot and the ST25TB02K answers after the command is issued.

ST25TB02K devices are allowed to answer only once during the anticollision sequence. Consequently, even if

there are several ST25TB02K devices present in the reader field, there will probably be a slot in which only

one ST25TB02K answers, allowing the reader to capture its Chip_ID. Using the Chip_ID, the reader can then

establish a communication channel with the identified ST25TB02K. The purpose of the anticollision sequence is

to allow the reader to select one ST25TB02K at a time.

The ST25TB02K is given an 8-bit Chip_ID value used by the reader to select only one among up to 256 tags

present within its field range. The Chip_ID is initialized with a random value during the Ready state, or after an

Initiate() command in the Inventory state.

The four least significant bits (b0 to b3) of the Chip_ID are also known as the Chip_slot_number. This 4-bit value is

used by the Pcall16() and Slot_marker() commands during the anticollision sequence in the Inventory state.

Figure 16. ST25TB02K Chip_ID description

8-bit Chip_ID

b0 to b3:Chip_slot_n umber

Each time the ST25TB02K receives a Pcall16() command, the Chip_slot_number is given a new 4-bit random

value. If the new value is 0000b, the ST25TB02K returns its whole 8-bit Chip_ID in its answer to the Pcall16()

command. The Pcall16() command is also used to define the slot number 0 of the anticollision sequence. When

the ST25TB02K receives the Slot_marker(SN) command, it compares its Chip_slot_number with the Slot_number

parameter (SN). If they match, the ST25TB02K returns its Chip_ID as a response to the command. If they do

not, the ST25TB02K does not answer. The Slot_marker(SN) command is used to define all the anticollision slot

numbers from 1 to 15.

ST25TB02K

Anticollision

DS11469 - Rev 7

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