HCS412_11 Datasheet, PDF (25/52 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder Burglar alarm systems

FIGURE 4-9: PROGRAM SEQUENCE - CONSECUTIVE WORDS
HCS412
Communication from reader to HCS412
Communication from HCS412 to reader
FIGURE 4-10: PROGRAMMING - VERIFICATION
01b Preamble
01
1 LFTE
Start
16-bit
Response
Write 18x16-bit
words total.
3LFTE Delay between
each 16-bit word
Start
Verify
Write 18x16-bit
words total.
Communication from reader to HCS412
Communication from HCS412 to reader
Approximately 1ms
delay before verify
begins.
Verify 18x16-bit
words total.
4.3.6 IFF CHALLENGE AND RESPONSE
The transponder reader sends one of four possible IFF
commands indicating which crypt key and which algo-
rithm to use to encrypt the challenge (Table 4-7).
The command is followed by the 32-bit challenge, typi-
cally a random number. The HCS412 encrypts the
challenge using the designated crypt key and algorithm
and responds with the 32-bit encrypted result. The
reader authenticates the response by comparing it to
the expected value.
The second crypt key and the seed value occupy the
same EEPROM storage area. To use the second crypt
key for IFF, the Seed Enable (SEED) and the Tempo-
rary Seed Enable (TMPSD) configuration options must
be disabled.
Note: If seed transmissions are not appropriately
disabled, the HCS412 will default to using
KEY1 for IFF.
TABLE 4-7: CHALLENGE AND RESPONSE COMMANDS
Command
Description
Expected data In
Response
10000
10001
10100
10101
IFF using key-1 and IFF algorithm
IFF using key-1 and HOP algorithm
IFF using key-2 and IFF algorithm
IFF using key-2 and HOP algorithm
32-Bit Challenge
32-Bit Challenge
32-Bit Challenge
32-Bit Challenge
32-Bit Response
32-Bit Response
32-Bit Response
32-Bit Response
© 2011 Microchip Technology Inc.
DS41099D-page 25