XRA00 datasheet(33/40 Pages) STMICROELECTRONICS

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XRA00

ANTI-COLLISION ALGORITHM

The PingID command divides a population of

XRA00 devices into eight sub-populations based

on their ID code by binning XRA00 responses into

eight separate time slices, or Bin Response Win-

dows. This binning provides the basis for an anti-

collision algorithm that probes the binary ID code

three bits at a time. Individual XRA00 devices can

be isolated from large populations in the field of

the Reader by issuing multiple PingID commands

to the field, analyzing the responses and eventual-

ly issuing the appropriate ScrollID command.

Figure 35. is a binary tree representation of the

first four LSBs in the XRA00 address space. Al-

though only the four first levels are shown, the bi-

nary tree structure applies to the entire XRA00 ID

code (96 bits for a Class-1b XRA00).

A PingID command with [PTR]=0, [LEN]=1 and

[VALUE]=0 will probe the right half of this tree (the

‘0’ branch) through the first four bits of the XRA00

memory.

Similarly, a PingID command with [PTR]=0,

[LEN]=1 and [VALUE]=1 will probe the left half of

this tree (the ‘1’ branch) through the first four bits

of the XRA00 memory.

For the PingID command with [PTR]=0, [LEN]=1

and [VALUE]=0, XRA00 devices whose LSBs are

0000b respond in bin 0, XRA00 devices whose

LSBs are 0010b respond in bin 1... and XRA00 de-

vices whose LSBs are 1110b respond in bin 7.

Readers can look for backscatter modulation from

the XRA00 devices in each of the bins and learn

about the XRA00 population even if collisions

make reading the 8 bits of data sent by the XRA00

devices difficult.

The presence of backscatter in a given bin merely

indicates that one or more XRA00 device(s)

match(es) the query. The bin number tells the

Reader what the next three MSBs of the XRA00

addresses will be. The bin contents also indicate

the next 5 bits of the responding XRA00 devices.

Figure 35. First Four LSBs as a Binary Tree

Anti-Collision Example

Consider two XRA00 devices having their lowest

address programmed with 0111b and 1011b, re-

spectively.

Query 1

[CMD] = 00001000b(PingID)

[PTR] = 00000000b(0x00)

[LEN] = 00000001b(0x01)

[VALUE] = 1b(1)

This command probes the left half of the tree

shown in Figure 35. The [VALUE] field of 1, match-

es the first bit of both XRA00 devices in the exam-

ple (0111b and 1011b), so that they will both

respond.

During the reply interval, the XRA00 devices mod-

ulate the next 8 bits of their address data that

come just after the matched portion on the higher

address side. The bin in which they modulate is

determined by the three LSBs of the data they

modulate. At the Reader, backscatter modulation

is observed in bins 3 (011b) and 5 (101b) (as

shown in Figure 36.). Since multiple XRA00 devic-

es can modulate in each of these bins, contention

may be observed but the Reader knows that there

are two distinct populations of XRA00 devices

present whose first four LSBs are 0111b and

1011b, respectively.

PTR

= 0

LEN

= 1

VALUE

= 1

PTR

= 0 LEN

= 1 VALUE

= 0

1111

1011

1101

1001

1110

1010

1100

1000

000

100

010

110

001

101

011

111

0111

0011

0101

0001

0110

0010

0100

0000

ai10773

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