HCS473 datasheet(41/68 Pages) MICROCHIP

41 / 68 page

 2002 Microchip Technology Inc.

Preliminary

DS40035C-page 39

HCS473

5.0

INTEGRATING THE HCS473

INTO A SYSTEM

Use of the HCS473 in a system requires a compatible

decoder. This decoder is typically a microcontroller with

a low frequency coil antenna and radio frequency

receiver. Example firmware routines that accept and

decrypt K

EELOQ transmissions can be found in Applica-

tion Notes and the K

EELOQ license disk.

5.1

Training the Receiver

In order for a transmitter to be used with a decoder, the

transmitter must first be ‘learned’. When a decoder

learns a transmitter, it is suggested that the decoder

stores the serial number and current synchronization

value in EEPROM. Some learning strategies have

been patented and care must be taken not to infringe

on them. The decoder must keep track of these values

for every transmitter that is learned (see Figure 5-1).

The maximum number of transmitters that can be

learned is limited only by the available EEPROM

memory. The decoder must also store the manufac-

turer’s code in order to learn a transmitter. This value

will not change in a typical system, so it is usually

stored as part of the microcontroller ROM code. Storing

the manufacturer’s code as part of the ROM code

improves security by keeping it off the external bus to

the EEPROM.

FIGURE 5-1:

TYPICAL LEARN

SEQUENCE

5.2

Decoder Operation

In a typical decoder operation (Figure 5-2), the key

generation on the decoder side is performed by taking

the serial number from a transmission and combining

that with the manufacturer’s code to create the same

secret key that was used by the transmitter. Once the

secret key is obtained, the rest of the transmission can

be decrypted. The decoder waits for a transmission

and immediately can check the serial number to

determine if it is a learned transmitter. If it is, the

encrypted portion of the transmission is decrypted

using the stored key. It uses the discrimination bits to

determine if the decryption was valid. If everything up

to this point is valid, the synchronization value

is evaluated.

Enter Learn

Mode

Wait for Reception

of a Valid Code & Seed

Generate Key

Use Generated Key

to Decrypt

Compare Discrimination

Value with Fixed Value

Equal

Wait for Reception

of Second Valid Code

Counters

Sequential

Exit

Learn

Unsuccessful

Yes

(Optional)

Learn Successful Store:

Serial Number

Encoder Key

Synchronization Counter

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