AN665

DS00665A-page 2

ã 1997 Microchip Technology Inc.

ENCODERS

technology to make each transmission by an encoder

unique. The encoder transmissions have two parts. The

first part changes each time the encoder is activated

and is called the code hopping part and is encrypted.

The second part is the unencrypted part of the trans-

mission, principally containing the encoder serial num-

ber identifying it to a decoder.

The code hopping contains function information, a dis-

crimination value, and a synchronization counter. This

information is encrypted by an encryption algorithm

before being transmitted. A 64-bit encryption key is

used by the encryption algorithm. If one bit in the data

that is encrypted changes, the result is that an average

of half the bits in the output will change. As a result, the

code hopping changes dramatically for each transmis-

sion and can not be predicted.

The synchronization information is used at the decoder

to determine whether a transmission is valid or is a rep-

etition of a previous transmission. Previous codes are

rejected to safeguard against code grabbers.

The HSC300 and HCS301 encoders transmit two over-

flow bits which may be used to extend the range of the

synchronization counter from 65,536 to 196,608 button

operations. The HCS300 and HCS301 encoders

include provision for four bits of function information

and two status bits in the fixed code portion of its trans-

mission. The two status bits indicate whether a

repeated transmission is being sent, and whether the

battery voltage is low.

The Microchip HCSXXX encoders all have the ability to

transmit a fixed seed. The seed value is programmed

into the encoder when the encoder is first initialized

along with the counters, key, serial number, and other

information. The seed length differs from encoder to

encoder, with the HCS300 and HCS301 having a 32-bit

seed.

LOGIC ‘0’

LOGIC ‘1’

Bit

Period

Preamble

Header

Encrypted Portion

of Transmission

Fixed Portion of

Transmission

Guard

Time

16-bit

Discrimination

bits

(10 bits)

Overflow

bits

(2 bits)

Button

Status

(4 bits)

Transmission Direction

32 bits of Encrypted Data

Encrypted using

Encrypted Code Data

Sync Value

28-bit Serial Number

Button

Status

(4 bits)

+

+

VLOW and

Repeat Status

(2 bits)

Serial Number and Button

2 bits

of Status

Fixed Code Data

Status (32 bits)