hcs410 Microchip Technology Inc., hcs410 Datasheet - Page 25

• Manufacturer Part Number: hcs410
• Description: Keeloq Code Hopping Encoder And Transponder Keeloq Code Hopping Encoder And Transponder
• Manufacturer: Microchip Technology Inc.

4.3.1
The K
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codes and are no longer valid.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.
FIGURE 4-6:
 2001 Microchip Technology Inc.
Note:
EE
SYNCHRONIZATION WITH DECODER
L
OQ
The synchronization method described in
this
implementation and because it is usually
implemented in firmware, it can be altered
to fit the needs of a particular system
technology features a sophisticated
Manufacturer Code
EEPROM Array
BASIC OPERATION OF A CODE HOPPING RECEIVER (DECODER)
section
Serial Number
Sync Counter
Encoder Key
Button Press
Information
is
only
Received Information
a
Serial Number
Check for
typical
Match
Preliminary
FIGURE 4-5:
Encrypted Data
Entire Window
rotates to eliminate
use of previously
used codes
Decryption
Algorithm
32 Bits of
K
EE
L
OQ
SYNCHRONIZATION WINDOW
Double
Operation
(32K Codes)
(32K Codes)
Blocked
Synchronization
Check for
Decrypted
Match
Counter
HCS410
Single Operation
Window (16 Codes)
DS40158E-page 25
Current
Position