hcs500 Microchip Technology Inc., hcs500 Datasheet - Page 7

• Manufacturer Part Number: hcs500
• Description: Keeloq Code Hopping Decoder
• Manufacturer: Microchip Technology Inc.

3.5
The K
sophisticated synchronization technique that does not
require the calculation and storage of future codes. The
technique securely blocks invalid transmissions while
providing transparent resynchronization to transmitters
inadvertently activated away from the receiver.
Figure 3-3 shows a 3-partition, rotating synchronization
window. The size of each window is optional but the
technique is fundamental. Each time a transmission is
authenticated, the intended function is executed and
the transmission’s synchronization counter value is
stored in EEPROM. From the currently stored counter
value there is an initial "Single Operation" forward win-
dow of 16 codes. If the difference between a received
synchronization counter and the last stored counter is
within 16, the intended function will be executed on the
single button press and the new synchronization
counter will be stored. Storing the new synchronization
counter value effectively rotates the entire synchroniza-
tion window.
FIGURE 3-3:
2002 Microchip Technology Inc.
EE
Synchronization with Decoder
(Evaluating the Counter)
L
OQ
technology patent scope includes a
Entire Window
rotates to eliminate
use of previously
used codes
SYNCHRONIZATION WINDOW
(resynchronization)
Double Operation
(32K Codes)
(32K Codes)
Blocked
Window
Window
A "Double Operation" (resynchronization) window fur-
ther exists from the Single Operation window up to 32K
codes forward of the currently stored counter value. It
is referred to as "Double Operation" because a trans-
mission with synchronization counter value in this win-
dow will require an additional, sequential counter
transmission prior to executing the intended function.
Upon receiving the sequential transmission the
decoder executes the intended function and stores the
synchronization counter value. This resynchronization
occurs transparently to the user as it is human nature
to press the button a second time if the first was unsuc-
cessful.
The third window is a "Blocked Window" ranging from
the double operation window to the currently stored
synchronization counter value. Any transmission with
synchronization counter value within this window will
be ignored. This window excludes previously used,
perhaps code-grabbed transmissions from accessing
the system.
Single Operation
(16 Codes)
Stored
Synchronization
Counter Value
Window
HCS500
DS40153C-page 7