HCS410 Datasheet, PDF (25/36 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER AND TRANSPONDER

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 portion of the transmission is decrypted using
the stored key. It uses the discrimination bits to deter-
mine if the decryption was valid. If everything up to this
point is valid, the synchronization counter value
is evaluated.
HCS410
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
© 1997 Microchip Technology Inc.
Preliminary
DS40158C-page 25