HCS410_11 Datasheet, PDF (24/46 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder 2-bit CRC, 2-bit queue

HCS410
4.3 CH Mode Decoder Operation
In a typical decoder operation (Figure 4-4), the key
generation on the decoder side is done by taking the
serial number from a transmission and combining that
with the manufacturer’s code to create the same
encoder key that is stored in the HCS410. Once the
encoder key is obtained, the rest of the transmission
can be decrypted. The decoder waits for a transmission
and immediately checks the serial number to determine
if it is a learned transmitter. If it is, the code hopping por-
tion of the transmission is decrypted using the stored
key. It uses the discrimination bits to determine if the
decryption was valid. If everything up to this point is
valid, the synchronization counter value is evaluated.
FIGURE 4-4: TYPICAL CH MODE
DECODER OPERATION
Start
No _Transmission
Received
?
Yes
No
Does
Serial Number
Match
?
Yes
Decrypt Transmission
Is
No Decryption
Valid
?
Yes
Is
No
Counter
Yes
Within 16
?
No
Is
No
Counter
Within 32K
?
Yes
Save Counter
in Temp Location
Execute
Command
and
Update
Counter
DS40158F-page 24
© 2011 Microchip Technology Inc.