HCS360-SLASHP datasheet(17/28 Pages) MICROCHIP

17 / 28 page

 2001 Microchip Technology Inc.

DS40152D-page 17

HCS360

7.2

Decoder Operation

In a typical decoder operation (Figure 7-2), the key

generation on the decoder side is done by taking the

serial number from a transmission and combining that

with the manufacturer’s code to create the same secret

key that was used by the transmitter. Once the secret

key is obtained, the rest of the transmission can be

decrypted. The decoder waits for a transmission and

immediately can check the serial number to determine

if it is a learned transmitter. If it is, it takes the encrypted

portion of the transmission and decrypts it using the

stored key It uses the discrimination bits to determine if

the decryption was valid. If everything up to this point is

valid, the synchronization value is evaluated.

FIGURE 7-2:

TYPICAL DECODER

OPERATION

7.3

Synchronization with Decoder

The KEELOQ technology features a sophisticated

synchronization technique (Figure 7-3) 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 formatted 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 transmitted synchronization 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 check 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 value is stored and the command

executed. If a transmitter has somehow gotten 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no longer valid.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 possibility of grab-

bing a previous code and retransmitting to gain entry.

FIGURE 7-3:

SYNCHRONIZATION WINDOW

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

only

typical

implementation. 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)

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