HCS412I MICROCHIP [Microchip Technology], HCS412I Datasheet - Page 21

• Manufacturer Part Number: HCS412I
• Description: KEELOQ Code Hopping Encoder and Transponder
• Manufacturer: MICROCHIP [Microchip Technology]

4.3.1
Multiple tokens in the same inductive field will simulta-
neously
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).
TABLE 4-3:
FIGURE 4-3:
Comms
Inductive
Comms
Command
Communication from reader to HCS412
Communication from HCS412 to reader
2002 Microchip Technology Inc.
RF
00000
28-bit Serial Number
ANTICOLLISION
respond
Activate
Field
Select HCS412, used if Anticolli-
sion enabled
DEVICE SELECT COMMAND
ANTICOLLISION - DEVICE SELECTION
ACK
to
Description
inductive
Command
Delay to
2 LF
Start
TE
commands.
Command
Command
The
Preliminary
The desired HCS412’s serial
Delay to
2 LF
Start
Expected data In
Serial
T
OTD
TE
number
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.
of Serial Number
starting with Bit 3.
1 to 25 bits of the
Most Sig X Bits
Serial Number,
starting with Bit 3.
1 to 25 bits of the
Serial Number,
2 LF
Start
Delay
2ms
TE
Encoder select Acknowledge if
serial number match
000b
Clock Serial
3 LSb’s
001b
Response
T
ESA
HCS412
Send ‘1’s to
increment 3LSb’s
010b
selection @ LSb = 011b
DS41099C-page 21
by ACK, indicating
4th ‘1’ interrupted
011b
Encoder
Select
ACK
ACK