Rainbow Electronics MAX66040 - Manual (Page 21)

MAX66040
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