HCS301
7.2
Decoder Operation
7.3
Synchronization with Decoder
(Evaluating the Counter)
Figure 7-2 summarizes normal decoder operation. The
decoder waits until a transmission is received. The
received serial number is compared to the EEPROM
table of learned transmitters to first determine if this
transmitter’s use is allowed in the system. If from a
learned transmitter, the transmission is decrypted
using the stored crypt key and authenticated via the
discrimination bits for appropriate crypt key usage. If
the decryption was valid the synchronization value is
evaluated.
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 7-3 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 7-2:
TYPICAL DECODER
OPERATION
Start
No
Transmission
Received
?
Yes
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.
Does
Serial Number
Match
No
?
Yes
Decrypt Transmission
Is
No
Decryption
Valid
?
Yes
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.
Execute
Command
and
Update
Counter
Is
Counter
Within 16
?
Yes
No
No
No
Is
Counter
Within 32K
?
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.
Yes
Save Counter
in Temp Location
DS21143B-page 16
2001 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
