HCS410_11 Datasheet, PDF (26/46 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder 2-bit CRC, 2-bit queue

HCS410
4.4 IFF Decoder Operation
In a typical IFF decoder, the key generation on the
decoder side is done by reading the serial number from
a token and combining that with the manufacturer’s
code to recreate the encoder key that is stored on the
token. The decoder polls for the presence of a token.
Once detected the decoder reads the serial number. If
the token has been learned, the decoder sends a chal-
lenge and reads the token’s response. The decoder
uses the encoder key stored in EEPROM and decrypt
response. The decrypt response is compared to the
challenge. If they match the appropriate output is acti-
vated.
FIGURE 4-7: TYPICAL IFF DECODER
OPERATION
Start
No
Token
Detected?
Yes
Read Serial
Number
No
Does
Serial Number
Match?
Yes
Send Challenge
and Read
Response
Decrypt the
Response
Does
No
Challenge &
Decrypt response
Match?
Yes
Execute Command
FIGURE 4-8: BASIC OPERATION OF AN IFF RECEIVER (DECODER)
EEPROM Array
IFF Key
Serial Number
KEELOQ®
IFF
Algorithm
Decrypted
Response
Manufacturer
Code
Serial Number
Response
Information read from HCS410
Check for
Match
Challenge
Written to HCS410
DS40158F-page 26
© 2011 Microchip Technology Inc.