HCS473_13 datasheet(26/68 Pages) MICROCHIP

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HCS473

DS40035D-page 26

Preliminary

 2000-2013 Microchip Technology Inc.

3.2.3.3

IFF CHALLENGE AND RESPONSE

The HCS473 can perform a 16-bit or 32-bit challenge

and response (IFF) based on the KEELOQ encryption

algorithm.

The transponder reader follows the 3-bit IFF command

with one of four possible options indicating a 16 or 32-

bit challenge and whether to use the encoder or tran-

sponder crypto key to create the response (Table 3-5).

The 3-bit option is followed by the appropriate 16 or 32-

bit

challenge;

typically

a random

number.

The

sequence ends with a CRC calculated over the com-

mand, option and challenge bits, (Figure 3-14).

The HCS473 encrypts the challenge using the desig-

nated crypto key and responds with a 32-bit result. The

reader authenticates the response by decrypting it and

verifying it matches the original challenge.

If 16-bit IFF is selected, the 32-bit response consists of

two copies of the 16-bit challenge.

RFRSP determines if the response will be transmitted

on the DATA pin. If enabled, the response will follow the

selected Encoder mode code hopping format with the

hopping code replaced with the 32-bit response. The

transmissions will contain a button code of ‘0000’.

LFRSP determines if the response will be transmitted

on the LC pins. The LC pin response will be the 32-bit

result, modulated PPM format.

If both RFRSP and LFRSP are enabled, the HCS473

will send the response on the DATA pin immediately fol-

lowed by the PPM response on the LC pins. Refer to

Section 3.2.1.7 for further response path details.

The next command must begin TCMD after the

response.

FIGURE 3-14:

IFF SEQUENCE (RF RESPONSE EXAMPLE)

TSF

ACK

CMD

OPT

CHAL

TCMD

‘0102’3 bits

16/32

CRC

2 bits

TIFF

RESPONSE

bits

Option

Command

CRC

TFINH

TIFF

RF Response (RFRSP=1)

16/32-Bit

Challenge

Optional Next

Command

TCMD

LF must remain on if following

with consecutive command

Wake Sequence

Previous Command

Response

Inductive In

(LC Pins)

RF Response

(DATA Pin)

TCMD

or if waiting for LF response

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