in the request and the CID in the device match. If the

DESELECT request does not include a CID, the

MAX66040 only responds to the request if its CID is

0000b.

The response frame to the DESELECT command is

identical to the request frame. The slave returns the

same data that it had received, confirming that the

slave addressed in the request has been transitioned to

the HALT sate.

Anticollision Examples

Probabilistic Anticollision

The master starts the anticollision process by issuing an

REQB or WUPB command. The WUPB command

involves any slave in the field with a matching AFI code.

The REQB command performs the same function, but is

ignored by slaves in the HALT state. Both commands

include the parameter N, which according to Table 15 is

used to set the probability of an ATQB response to 1/N.

If N = 1, all participating slaves respond with the ATQB

response. If N is greater than one, then each slave

selects a random number R in the range of 1 to N. If a

slave happens to choose R = 1, then it responds with

ATQB. If R is greater than 1, then the slave waits for

another REQB or WUPB command, which causes the

participating slaves to choose a new random number R.

The ATQB response contains a field named PUPI,

which is used to direct commands to a specific slave

during the anticollision process. When the master

receives an ATQB response, it should issue a matching

HLTB command to halt the slave or issue a matching

ATTRIB command to assign a CID and place the slave

in the ACTIVE state. If this is not done, the slaves con-

tinue to participate in the anticollision process. A slave

in the ACTIVE state ignores all REQB, WUPB, SLOT-

MARKER, ATTRIB, and HLTB commands, but responds

to the DESELECT command.

An ATQB response received with a CRC error indicates

a collision because two or more slaves have responded

at the same time. With probabilistic anticollision, the

master must issue another REQB command to cause

the slaves in the field that are not in the HALT or

ACTIVE state to select a new random number R. If one

of the slaves has chosen R = 1, it responds with ATQB.

A REQB without ATQB response does not guaran-

tee that all slaves in the field have been identified.

Figure 30 shows an example of the time-slot anticolli-

sion, assuming that there are four slaves in IDLE state in

the field. The process begins with the master sending

an REQB request with N = 1, which forces all slaves to

respond with ATQB, resulting in a collision. Knowing that

slaves are present, the master now sends REQB with N

= 8. This causes all slaves to select a random number in

the range of 1 to 8. Only the slave that has chosen R = 1

responds, which is slave C in the example. Knowing that

there are more slaves in the field, the master continues

issuing REQB commands, which in the example, even-

tually identifies all slaves. Due to its statistical nature,

probabilistic anticollision is less likely to find every slave

in the field than the time-slot anticollision.

ISO/IEC 14443 Type B-Compliant

Secure Memory

30

______________________________________________________________________________________

ABRIDGED DATA SHEET

TESTING FOR SLAVES

ATTEMPT 1

ATTEMPT 2

ATTEMPT 3

ATTEMPT 4

ATTEMPT 5

ATTEMPT 6

MASTER

REQB

(N = 1)

REQB

(N = 8)

REQB

(N = 8)

REQB

(N = 8)

REQB

(N = 8)

REQB

(N = 8)

REQB

(N = 8)

SLAVE A

ATQB

(R = 3)

(R = 7)

(R = 1) ATQB

(R = 3)

(R = 6)

(R = 8)

SLAVE B

ATQB

(R = 6)

(R = 4)

(R = 8)

(R = 8)

(R = 5)

(R = 1) ATQB

SLAVE C

ATQB

(R = 1) ATQB (R = 8)

(R = 2)

(R = 4)

(R = 3)

(R = 4)

SLAVE D

ATQB

(R = 2)

(R = 1) ATQB (R = 5)

(R = 8)

(R = 4)

(R = 2)

Figure 30. Probabilistic Anticollision Example