HCS360 datasheet(18/40 Pages) MICROCHIP

18 / 40 page

HCS360

DS40152F-page 18

7.2

Decoder Operation

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.

FIGURE 7-2:

TYPICAL DECODER

OPERATION

7.3

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 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 coun-

ter will be stored. Storing the new synchronization

counter value effectively rotates the entire synchroniza-

tion 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.

Transmission

Received

Does

Serial Number

Match

Decrypt Transmission

Decryption

Valid

Counter

Within 16

Counter

Within 32K

Update

Counter

Execute

Command

Save Counter

in Temp Location

Start

Yes

and

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.

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