HCS512_11 Datasheet, PDF (7/34 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder system and high security

4.2 Validation of Codes
The decoder waits for a transmission and checks the
serial number to determine if the transmitter has been
learned. If learned, the decoder decrypts the encrypted
portion of the transmission using the crypt key. It uses
the discrimination bits to determine if the decryption
was valid. If everything up to this point is valid, the
synchronization value is evaluated.
4.3 Validation Steps
Validation consists of the following steps:
• Search EEPROM to find the Serial Number
Check Value Match
• Decrypt the Hopping Code
• Compare the 10 bits of discrimination value with
the lower 10 bits of serial number
• Check if the synchronization counter falls within
the first synchronization window.
• Check if the synchronization counter falls within
the second synchronization window.
• If a valid transmission is found, update the syn-
chronization counter, else use the next transmitter
block and repeat the tests.
HCS512
FIGURE 4-2: DECODER OPERATION
Start
No Transmission
Received
?
Yes
No
Does
Ser # Check Val
Match
?
Yes
Decrypt Transmission
Is
No _Decryption
Valid
?
Yes
Is
Counter
Yes
Within 16
?
No
Is
No Counter
Within 32K
?
Yes
Save Counter
in Temp Location
Execute
Command
and
Update
Counter
© 2011 Microchip Technology Inc.
DS40151E-page 7