HCS410 datasheet(4/36 Pages) MICROCHIP

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HCS410

DS40158C-page 4

Preliminary

1.2

KEELOQ Code Hopping Encoders

When the HCS410 is used as a code hopping encoder

device, it is ideally suited to 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.

Most keyless entry systems transmit the same code

from a transmitter every time a button is pushed. The

relative number of 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

retransmits it later or a device that scans all possible

combinations until the correct one is found.

The HCS410 employs the KEELOQ code hopping tech-

nology and an 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 69 bits,

virtually eliminates the use of code ‘grabbing’ or code

‘scanning’.

The HCS410 has a small EEPROM array which must

be loaded with several parameters before use. The

most important of these values are:

• A 28/32-bit serial number which is meant to be

unique for every encoder

• 64-bit seed value

• A 64-bit encoder key that is generated at the time

of production

• A 16-bit synchronization counter value.

• Conﬁguration options

The 16-bit synchronization counter value is the basis

for the transmitted code changing for each transmis-

sion, and is updated each time a button is pressed.

Because of the complexity of the code hopping encryp-

tion algorithm, a change in one bit of the synchroniza-

tion counter value will result in a large change in the

actual transmitted code.

Once the encoder detects that a button has been

pressed, the encoder reads the button and updates the

synchronization counter. The synchronization counter

value, the function bits, and the discrimination value are

then combined with the encoder key in the encryption

algorithm, and the output is 32 bits of encrypted infor-

mation (Figure 1-1). The code hopping portion pro-

vides up to four billion changing code combinations.

This data will change with every button press, hence, it

is referred to as the code hopping portion of the code

word.

The 32-bit code hopping portion is combined with the

button information and the serial number to form the

code word transmitted to the receiver. The code word

format is explained in detail in Section 2.2.

FIGURE 1-1:

BASIC OPERATION OF A CODE HOPPING TRANSMITTER (ENCODER)

KEELOQ

Algorithm

Button Press

Information

Encryption

EEPROM Array

32 Bits of

Encrypted Data

Serial Number

Transmitted Information

Encoder Key

Sync Counter

Serial Number

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