HCS500T-ISLASHST datasheet(6/42 Pages) MICROCHIP

6 / 42 page

HCS500

DS40000153E-page 6

 2001-2015 Microchip Technology Inc.

The following checks are performed by the decoder to

determine if the transmission is valid during learn:

• The first code word is checked for bit integrity

• The second code word is checked for bit integrity

• The crypt key is generated according to the

selected algorithm

• The hopping code is decrypted

• The discrimination value is checked

• If all the checks pass, the key, serial number

check value, and synchronization counter values

are stored in EEPROM memory

Figure 3-1 shows a flow chart of the learn sequence.

FIGURE 3-1:

LEARN SEQUENCE

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

transmission and decrypts it, using the crypt 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.

3.4

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.

Enter Learn

Mode

Wait for Reception

of Second

Compare Discrimination

Value with Serial Number

Use Generated Key

to Decrypt

Equal?

Sync. counter value

crypt key

Exit

Learn successful. Store:

Learn

Unsuccessful

Yes

Wait for Reception

of a Valid Code

Non-Repeated

Valid Code

Generate Key

from Serial Number/

Seed Value

Serial number check value

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