HCS200

DS40138A-page 10

Preliminary

© 1996 Microchip Technology Inc.

7.0

INTEGRATING THE HCS200

INTO A SYSTEM

Use of the HCS200 in a system requires a compatible

decoder. This decoder is typically a microcontroller with

compatible firmware. Microchip will provide (via a

license agreement) firmware routines that accept

transmissions from the HCS200 and decrypt the

hopping code portion of the data stream. These

routines provide system designers the means to

develop their own decoding system.

7.1

Learning a transmitter to a receiver

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

transmitter must first be ‘learned’. Several learning

strategies can be followed in the decoder implementa-

tion. When a transmitter is learned to a decoder, it is

suggested that the decoder stores the serial number

and current synchronization value in EEPROM. The

decoder must keep track of these values for every

transmitter that is learned (Figure 7-1). The maximum

number of transmitters that can be learned is only a

function of how much EEPROM memory storage is

available. The decoder must also store the manufac-

turer’s code in order to learn a transmission transmitter,

although 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 is also better for security reasons.

It must be stated that some learning strategies have

been patented and care must be taken not to infringe.

FIGURE 7-1:

TYPICAL LEARN SEQUENCE

Enter Learn

Mode

Wait for Reception

of a Valid Code

Generate Key

from Serial Number

Use Generated Key

to Decrypt

Compare Discrimination

Value with Fixed Value

Equal

Wait for Reception

of Second Valid Code

Compare Discrimination

Value with Fixed Value

Use Generated Key

to Decrypt

Equal

Counters

Encryption key

Serial number

Synchronization counter

Sequential

?

?

?

Exit

Learn successful.

Learn

Unsuccessful

No

No

No

Yes

Yes

Yes

Store: