HCS500 datasheet(6/40 Pages) MICROCHIP

6 / 40 page

HCS500

DS40153D-page 6

3.3

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 crypt key. It uses the dis-

crimination value to determine if the decryption was

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

synchronization counter value is evaluated.

3.4

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

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

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