HCS200
7.2
Decoder operation
7.3
Synchronization with Decoder
In a typical decoder operation (Figure 7-2), the key gen-
eration 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.
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 trans-
mitted synchronization value is stored in temporary
location and it goes back to waiting for another trans-
mission. 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 single opera-
tion ‘window’, but is now back in sync, so the new syn-
chronization 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 re-learned. 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no longer valid.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 re-transmitting to gain entry.
FIGURE 7-2: TYPICAL DECODER
OPERATION
Start
No
Transmission
Received
?
Yes
Note: The synchronization method described in
this section is only a typical implementation
and because it is usually implemented in
firmware, it can be altered to fit the needs
of a particular system
Does
Serial Number
Match
No
?
Yes
Decrypt Transmission
FIGURE 7-3: SYNCHRONIZATION WINDOW
Entire Window
rotates to eliminate
use of previously
used codes
Is
No
Decryption
Valid
?
Blocked 32K
Codes
Yes
Current
Position
Execute
Command
and
Update
Counter
Is
Counter
Within 16
?
Yes
No
No
Open 32K
Codes
No
Current window
of 16 codes
Is
Counter
Within 32K
?
Yes
Save Counter
in Temp Location
1996 Microchip Technology Inc.
Preliminary
DS40138A-page 11
MICROCHIP [ MICROCHIP ]
