HCS410 datasheet(27/36 Pages) MICROCHIP

27 / 36 page

HCS410

Preliminary

DS40158C-page 27

4.4

IFF Decoder Operation

In a typical IFF decoder, the key generation on the

decoder side is done by reading the serial number from

a token and combining that with the manufacturer’s

code to recreate the encoder key that is stored on the

token. The decoder polls for the presence of a token.

Once detected the decoder reads the serial number. If

the token has been learned, the decoder sends a chal-

lenge and reads the token’s response. The decoder

uses the encoder key stored in EEPROM and decrypt

response. The decrypt response is compared to the

challenge. If they match the appropriate output is acti-

vated.

FIGURE 4-7:

TYPICAL IFF DECODER

OPERATION

FIGURE 4-8:

BASIC OPERATION OF AN IFF RECEIVER (DECODER)

Start

Token

Detected?

Read Serial

Does

Serial Number

Match?

Send Challenge

and Read

Decrypt the

Response

Does

Challenge &

Match?

Execute Command

Yes

Response

Number

Decrypt response

IFF Key

Serial Number

KEELOQ

IFF

Algorithm

Decrypted

EEPROM Array

Manufacturer

Code

Serial Number

Response

Check for

Match

Response

Written to HCS410

Challenge

Information read from HCS410

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