SRI4K datasheet(22/45 Pages) STMICROELECTRONICS

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Anticollision

SRI4K

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Anticollision

The SRI4K 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 SRI4K individually, and access its memory. The anticollision sequence is managed by

the reader through a set of commands described in Section 5: SRI4K operation:

●

Initiate()

●

Pcall16()

●

Slot_marker().

The reader is the master of the communication with one or more SRI4K device(s). It initiates

the tag communication activity by issuing an Initiate(), Pcall16() or Slot_marker() command

to prompt the SRI4K to answer. During the anticollision sequence, it might happen that two

or more SRI4K devices respond simultaneously, so causing a collision. The command set

allows the reader to handle the sequence, to separate SRI4K transmissions into different

time slots. Once the anticollision sequence has completed, SRI4K communication is fully

under the control of the reader, allowing only one SRI4K to transmit at a time.

The Anticollision scheme is based on the definition of time slots during which the SRI4K

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 SRI4K answers after the command is issued. SRI4K devices are allowed to answer

only once during the anticollision sequence. Consequently, even if there are several SRI4K

devices present in the reader field, there will probably be a slot in which only one SRI4K

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

establish a communication channel with the identified SRI4K. The purpose of the

anticollision sequence is to allow the reader to select one SRI4K at a time.

The SRI4K 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 (b

0 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 20.

SRI4K Chip_ID description

Each time the SRI4K receives a Pcall16() command, the Chip_slot_number is given a new

4-bit random value. If the new value is 0000b, the SRI4K 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 SRI4K receives the Slot_marker(SN)

command, it compares its Chip_slot_number with the Slot_number parameter (SN). If they

match, the SRI4K returns its Chip_ID as a response to the command. If they do not, the

SRI4K does not answer. The Slot_marker(SN) command is used to define all the

anticollision slot numbers from 1 to 15.

ai07668b

8-bit Chip_ID

b0 to b3: Chip_slot_number

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