HCS500_15 Datasheet, PDF (7/42 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder

FIGURE 3-2: DECODER OPERATION
Start
No _Transmission
Received?
Yes
No
Does
Ser # Check Val
Match?
Yes
Decrypt Transmission
No
Is
decryption
valid?
Yes
Is
counter within
Yes
16?
No
No
Is
counter within
16K?
Execute
Command
and
Update
Counter
Yes
Save Counter
in Temp Location
HCS500
3.5 Synchronization with Decoder
(Evaluating the Counter)
The KEELOQ technology patent scope includes a
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
window 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 synchronization window.
A “Double Operation” (resynchronization) window
further 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
transmission with synchronization counter value in this
window 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
unsuccessful.
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.
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DS40000153E-page 7