© 1996 Microchip Technology Inc.

Preliminary

DS21143A-page 13

HCS301

7.2

Decoder operation

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

eration 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

?

Transmission

Received

Does

Serial Number

Match

?

Decrypt Transmission

Is

Decryption

Valid

?

Is

Counter

Within 16

?

Is

Counter

Within 32K

?

Update

Counter

Execute

Command

Save Counter

in Temp Location

Start

No

No

No

No

Yes

Yes

Yes

Yes

Yes

No

and

No

7.3

Synchronization with Decoder

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 trans-

mitted synchronization value is stored in temporary

location and it goes back to waiting for another trans-

mission. 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 single opera-

tion ‘window’, but is now back in sync, so the new syn-

chronization 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 re-learned. 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no longer valid.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 re-transmitting to gain entry.

FIGURE 7-3:

SYNCHRONIZATION WINDOW

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

Blocked 32K

Codes

Open 32K

Codes

Entire Window

rotates to eliminate

use of previously

used codes

Current window

of 16 codes

Current

Position