HCS200

DS40138A-page 2

Preliminary

© 1996 Microchip Technology Inc.

The HCS200 operates over a wide voltage range of

3.5 volts to 13.0 volts and has three button inputs in an

8-pin configuration. This allows the system designer the

freedom to utilize up to seven functions. The only

components required for device operation are the but-

tons and RF circuitry, allowing a very low system cost.

1.0

SYSTEM OVERVIEW

Key Terms

• Manufacturer’s code — a 64-bit word, unique to

each manufacturer, used to produce a unique

encryption key in each transmitter (encoder).

• Encryption Key — a unique 64-bit key generated

and programmed into the encoder during the

manufacturing process. The encryption key

controls the encryption algorithm and is stored in

EEPROM on the encoder device.

1.1

Learn

The HCS product family facilitates several learn strate-

gies to be implemented on the decoder. The following

are examples of what can be done. It must be pointed

out that their exists some third-party patents on learn-

ing strategies and implementation.

1.1.1

NORMAL LEARN

The receiver uses the same information that is transmit-

ted during normal operation to derive the transmitter’s

secret key, decrypt the discrimination value and the

synchronization counter.

1.1.2

SECURE LEARN*

The transmitter is activated through a special button

combination to transmit a stored 48-bit value (random

seed) that can be used for key generation or be part of

the key. Transmission of the random seed can be dis-

abled after learning is completed.

The HCS200 is a code hopping encoder device that is

designed specifically for keyless entry systems,

primarily for vehicles and home garage door openers. It

is meant to be a cost-effective, yet secure solution to

such systems. The encoder portion of a keyless entry

system is meant to be carried by the user and operated

to gain access to a vehicle or restricted area. The

HCS200 requires very few external components

(Figure 2-1).

Most low-end keyless entry systems transmit the same

code from a transmitter every time a button is pushed.

The number of possible code combinations for a low

end system is also a relatively small number. These

shortcomings provide the means for a sophisticated

thief to create a device that ‘grabs’ a transmission and

re-transmits it later, or a device that scans all possible

combinations until the correct one is found.

encryption algorithm to achieve a high level of security.

Code hopping is a method by which the code

transmitted from the transmitter to the receiver is

different every time a button is pushed. This method,

coupled with a transmission length of 66 bits, virtually

eliminates the use of code ‘grabbing’ or code

‘scanning’.

As indicated in the block diagram on page one, the

HCS200 has a small EEPROM array which must be

loaded with several parameters before use. The most

important of these values are:

• A 28-bit serial number which is meant to be

unique for every encoder

• A 16-bit configuration value

• An encryption key that is generated at the time of

production

• A 16-bit synchronization value

The serial number for each transmitter is programmed

by the manufacturer at the time of production. The

generation of the encryption key is done using a key

generation algorithm (Figure 1-1). Typically, inputs to

the key generation algorithm are the serial number of

the transmitter and a 64-bit manufacturer’s code. The

manufacturer’s

code

is

chosen

by

the

system

manufacturer and must be carefully controlled. The

manufacturer’s code is a pivotal part of the overall

system security.

FIGURE 1-1:

CREATION AND STORAGE OF ENCRYPTION KEY DURING PRODUCTION

Transmitter

Manufacturer’s

Serial Number

Code

Encryption

Key

Key

Generation

Algorithm

Serial Number

Encryption Key

Sync Counter

.

.

.

HCS200 EEPROM Array

* Code Hopping learn patents pending.