HCS500-IP datasheet(6/24 Pages) MICROCHIP

6 / 24 page

HCS500

DS40153B-page 6

Preliminary

3.2

Validation of Codes

The decoder waits for a transmission and checks the

serial number to determine if it is a learned transmitter.

If it is, it takes the code hopping portion of the transmis-

sion and decrypts it, using the encoder key. It uses the

discrimination value to determine if the decryption was

valid. If everything up to this point is valid, the

synchronization counter value is evaluated.

3.3

Validation Steps

Validation consists of the following steps:

Search EEPROM to ﬁnd the Serial Number

Check Value Match

Decrypt the Hopping Code

Compare the 10 bits of the discrimination value

with the lower 10 bits of serial number

Check if the synchronization counter value falls

within the ﬁrst synchronization window.

Check if the synchronization counter value falls

within the second synchronization window.

If a valid transmission is found, update the

synchronization counter, else use the next

transmitter block, and repeat the tests.

FIGURE 3-2:

DECODER OPERATION

3.4

Synchronization with Decoder

The KEELOQ technology features a sophisticated

synchronization technique (Figure 3-3) which does not

require the calculation and storage of future codes. If

the stored synchronization counter value for that

particular transmitter and the synchronization counter

value that was just decrypted are within a formatted

window of 16, the counter is stored, and the command

is executed. If the synchronization counter value was

not within the single operation window, but is within the

double operation window of the 16K window, the

transmitted synchronization counter value is stored in a

temporary location, and the decoder goes back to wait-

ing for another transmission. When the next valid

transmission is received, it will check the new

synchronization counter value with the one in tempo-

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

assumed that the counter had just gotten out of the

single operation ‘window’, but is now back in synchroni-

zation, so the new synchronization counter value is

stored, and the command is 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 become

part of the ‘blocked’ (48K) codes and are no longer

valid. This eliminates the possibility of grabbing a previ-

ous code and retransmitting to gain entry.

FIGURE 3-3:

SYNCHRONIZATION WINDOW

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

Blocked

Entire Window

rotates to eliminate

use of previously

used codes

Current

Position

(48K Codes)

Double

Operation

(16K Codes)

Single

Operation

Window

(16 Codes)

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