HCS473 Datasheet, PDF (28/68 Pages) Microchip Technology – Code Hopping Encoder and Transponder

HCS473
3.2.3.3 IFF CHALLENGE AND RESPONSE
The HCS473 can perform a 16-bit or 32-bit challenge
and response (IFF) based on the KEELOQ encryption
algorithm.
The transponder reader follows the 3-bit IFF command
with one of four possible options indicating a 16 or 32-
bit challenge and whether to use the encoder or tran-
sponder crypto key to create the response (Table 3-5).
The 3-bit option is followed by the appropriate 16 or 32-
bit challenge; typically a random number. The
sequence ends with a CRC calculated over the com-
mand, option and challenge bits, (Figure 3-14).
The HCS473 encrypts the challenge using the desig-
nated crypto key and responds with a 32-bit result. The
reader authenticates the response by decrypting it and
verifying it matches the original challenge.
If 16-bit IFF is selected, the 32-bit response consists of
two copies of the 16-bit challenge.
RFRSP determines if the response will be transmitted
on the DATA pin. If enabled, the response will follow the
selected Encoder mode code hopping format with the
hopping code replaced with the 32-bit response. The
transmissions will contain a button code of ‘0000’.
LFRSP determines if the response will be transmitted
on the LC pins. The LC pin response will be the 32-bit
result, modulated PPM format.
If both RFRSP and LFRSP are enabled, the HCS473
will send the response on the DATA pin immediately fol-
lowed by the PPM response on the LC pins. Refer to
Section 3.2.1.7 for further response path details.
The next command must begin TCMD after the
response.
FIGURE 3-14:
IFF SEQUENCE (RF RESPONSE EXAMPLE)
Wake Sequence
or
Previous Command
Response
16/32
‘010₂’ 3 bits bits 2 bits
TCMD _{CMD OPT CHAL CRC} TIFF _RESPONSE TCMD
Command Option
16/32-Bit
Challenge
CRC
Optional Next
Command
TSF
Inductive In
(LC Pins)
RF Response
(DATA Pin)
TCMD
ACK
TCMD
LF must remain on if following
with consecutive command
or if waiting for LF response
TFINH
TIFF
RF Response (RFRSP=1)
DS40035C-page 26
Preliminary
 2002 Microchip Technology Inc.