HCS500T-ISLASHST datasheet(7/42 Pages) MICROCHIP

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 2001-2015 Microchip Technology Inc.

DS40000153E-page 7

HCS500

FIGURE 3-2:

DECODER OPERATION

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.

Transmission

Received?

Does

Ser # Check Val

Match?

Decrypt Transmission

decryption

valid?

counter within

16?

counter within

16K?

Update

Counter

Execute

Command

Save Counter

in Temp Location

Start

Yes

and

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