HCS360_01 Datasheet, PDF (17/28 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder

7.2 Decoder Operation
In a typical decoder operation (Figure 7-2), 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 secret
key that was used by the transmitter. Once the secret
key is obtained, the rest of the transmission can be
decrypted. The decoder waits for a transmission and
immediately can check the serial number to determine
if it is a learned transmitter. If it is, it takes the encrypted
portion of the transmission and decrypts it 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 value is evaluated.
FIGURE 7-2: TYPICAL DECODER
OPERATION
Start
No _Transmission
Received
?
Yes
No
Does
Serial Number
Match
?
Yes
Decrypt Transmission
Is
No Decryption
Valid
?
Yes
No
Is
Counter
Within 16
Execute
Yes ^Command
and
?
Update
Counter
No
Is
No
Counter
Within 32K
?
Yes
Save Counter
in Temp Location
HCS360
7.3 Synchronization with Decoder
The KEELOQ technology features a sophisticated
synchronization technique (Figure 7-3) which does not
require the calculation and storage of future codes. If
the stored counter value for that particular transmitter
and the counter value that was just decrypted are
within a formatted window of say 16, the counter is
stored and the command is executed. If the counter
value was not within the single operation window, but is
within the double operation window of say 32K window,
the transmitted synchronization value is stored in tem-
porary location and it goes back to waiting for another
transmission. When the next valid transmission is
received, it will check the new value with the one in
temporary storage. If the two values are sequential, it is
assumed that the counter had just gotten out of the sin-
gle operation ‘window’, but is now back in sync, so the
new synchronization value is stored and the command
executed. If a transmitter has somehow gotten out of
the double operation window, the transmitter will not
work and must be relearned. Since the entire window
rotates after each valid transmission, codes that have
been used are part of the ‘blocked’ (32K) codes and are
no longer valid. This eliminates the possibility of grab-
bing a previous code and retransmitting to gain entry.
Note:
The synchronization method described in
this section is only a typical
implementation. It is usually implemented
in firmware, it can be altered to fit the
needs of a particular system
FIGURE 7-3: SYNCHRONIZATION WINDOW
Entire Window
rotates to eliminate
use of previously
used codes
Blocked
(32K Codes)
Current
Position
Double
Operation
(32K Codes)
Single Operation
Window (16 Codes)
 2001 Microchip Technology Inc.
DS40152D-page 17