HCS410 datasheet(25/46 Pages) MICROCHIP

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HCS410

DS40158F-page 25

4.3.1

SYNCHRONIZATION WITH DECODER

The KEELOQ technology features a sophisticated

synchronization technique (Figure 4-5) which does not

require the calculation and storage of future codes. If

the stored counter value for that particular transmitter

and the counter value that was just decrypted are

within a window of say 16, the counter is stored and the

command is executed. If the counter value was not

within the single operation window, but is within the

double operation window of say 32K window, the trans-

mitted synchronization counter value is stored in tem-

porary location and it goes back to waiting for another

transmission. When the next valid transmission is

received, it will compare the new value with the one in

temporary storage. If the two values are sequential, it is

assumed that the counter had just gotten out of the sin-

gle operation ‘window’, but is now back in sync, so the

new synchronization counter value is stored and the

command executed. If a transmitter has somehow got-

ten out of the double operation window, the transmitter

will not work and must be relearned. Since the entire

window rotates after each valid transmission, codes

that have been used are part of the ‘blocked’ (32K)

codes and are no longer valid. This eliminates the pos-

sibility of grabbing a previous code and retransmitting

to gain entry.

FIGURE 4-5:

SYNCHRONIZATION WINDOW

FIGURE 4-6:

BASIC OPERATION OF A CODE HOPPING RECEIVER (DECODER)

Note:

The synchronization method described in

this

section

only

typical

implementation and because it is usually

implemented in firmware, it can be altered

to fit the needs of a particular system

Blocked

Entire Window

rotates to eliminate

use of previously

used codes

Current

Position

(32K Codes)

Double

Operation

(32K Codes)

Single Operation

Window (16 Codes)

Button Press

Information

EEPROM Array

Encoder Key

32 Bits of

Encrypted Data

Serial Number

Received Information

Decrypted

Synchronization

Counter

Check for

Match

Check for

Match

KEELOQ®

Algorithm

Decryption

Sync Counter

Serial Number

Manufacturer Code

40158F.book Page 25 Wednesday, June 1, 2011 10:36 AM

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