HCS410_11 Datasheet, PDF (25/46 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder 2-bit CRC, 2-bit queue

HCS410
4.3.1 SYNCHRONIZATION WITH DECODER
The KEELOQ technology features a sophisticated
synchronization technique (Figure 4-5) 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 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 counter 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 compare 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 counter value is stored and the
command executed. If a transmitter has somehow got-
ten 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codes and are no longer valid.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 pos-
sibility of grabbing a previous code and retransmitting
to gain entry.
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
FIGURE 4-5: 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)
FIGURE 4-6: BASIC OPERATION OF A CODE HOPPING RECEIVER (DECODER)
EEPROM Array
Encoder Key
Sync Counter
KEELOQ®
Decryption
Algorithm
Check for
Match
Decrypted
Synchronization
Counter
Serial Number
Check for
Match
Manufacturer Code
Button Press
Information
Serial Number
32 Bits of
Encrypted Data
Received Information
© 2011 Microchip Technology Inc.
DS40158F-page 25