HCS500 Datasheet, PDF(6/24 Page) Microchip Technology

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HCS500 Datasheet, PDF (6/24 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

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HCS500

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:

1. Search EEPROM to ï¬nd the Serial Number

Check Value Match

2. Decrypt the Hopping Code

3. Compare the 10 bits of the discrimination value

with the lower 10 bits of serial number

4. Check if the synchronization counter value falls

within the ï¬rst synchronization window.

5. Check if the synchronization counter value falls

within the second synchronization window.

6. 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

Start

No Transmission

Received?

Yes

Does

Ser # Check Val

Match?

Yes

Decrypt Transmission

decryption

valid?

Yes

counter within

Yes

16?

counter within

16K?

Execute

Command

and

Update

Counter

Yes

Save Counter

in Temp Location

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 âwindowâ, 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 âblockedâ (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

Entire Window

rotates to eliminate

use of previously

used codes

Blocked

(48K Codes)

Current

Position

Single

Operation

Window

(16 Codes)

Double

Operation

(16K Codes)

DS40153B-page 6

Preliminary

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