HCS412 Datasheet, PDF (21/44 Pages) Microchip Technology – KEELOQ Code Hopping Encoder and Transponder

HCS412
4.3.1 ANTICOLLISION
Multiple tokens in the same inductive field will simulta-
neously respond to inductive commands. The
responses will collide making token authentication
impossible. Enabling anticollision allows addressing of
an individual token, regardless how many tokens are in
the field.
The HCS412 method is that all tokens trained to a
given vehicle will have the same 25 MSb’s of their serial
number. The serial numbers of up to 8 tokens trained to
access a given vehicle will differ only in the 3 LSb’s.
Think of the 25 MSb’s of the HCS412's serial number
as the vehicle ID and the 3 LSb’s as the token ID. The
vehicle ID associates the token with a given vehicle
and the token ID makes it a uniquely addressable
(selectable) 1 of 8 possible tokens authorized to access
the vehicle.
The transponder reader addresses an individual token,
HCS412, by sending a ‘SELECT ENCODER’ command.
The command is followed by from 1 to 25 bits of the
HCS412's serial number, starting with bit 3 (Least Sig-
nificant bit first) (Figure 4-3).
Clocking out ‘1’s then increments the 3 LSb’s, the first
‘1’ setting the bits to 000b. When the value matches
the 3 LSb’s of a token, the token responds with an
Encoder Select Acknowledge. The reader must halt
clocking out further ‘1’s or risk selecting multiple
tokens. Any remaining tokens in the field will be unse-
lected, responding only if a new device selection
sequence selects them. Removing the field will also
RESET a selected/unselected state if removed long
enough to result in a device RESET.
The ability to isolate a single HCS412 for communica-
tion greatly depends on the number of Most Significant
serial number bits included in the device selection
sequence. The more serial number bits sent, the more
narrow the device selection. All bits not transmitted are
treated as wildcards. Sending only 1 bit, bit 3 as a ‘0’,
will only narrow the number of tokens allowed to
respond to all with bit 3 equal to ‘0’. When the transpon-
der reader sends the full 25 MSb’s of the serial number,
it narrows all possible tokens down to only those
trained to the vehicle - only those tokens whose serial
number’s 25 MSb’s match.
TABLE 4-3: DEVICE SELECT COMMAND
Command
Description
Expected data In
Response
00000 Select HCS412, used if Anticolli- The desired HCS412’s serial Encoder select Acknowledge if
sion enabled
number
serial number match
FIGURE 4-3: ANTICOLLISION - DEVICE SELECTION
Activate
Field ^ACK
Delay to
Command
Command
Delay to
Serial
TOTD
Most Sig X Bits
of Serial Number ^Delay
1 to 25 bits of the
Serial Number,
2ms
starting with Bit 3.
Clock Serial
3 LSb’s
ACK
4th ‘1’ interrupted
by ACK, indicating
selection @ LSb = 011b
000b 001b 010b 011b
Inductive
Comms
RF
Comms
2 LFTE
Start
Communication from reader to HCS412
Communication from HCS412 to reader
Command
2 LFTE
Start
2 LFTE
Start
1 to 25 bits of the
Serial Number,
starting with Bit 3.
TESA
Encoder
Select
ACK
Send ‘1’s to
increment 3LSb’s
28-bit Serial Number
© 2002 Microchip Technology Inc.
Preliminary
DS41099C-page 21