LICAL-DEC-HS001 데이터 시트보기 (PDF) - Unspecified

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LICAL-DEC-HS001

HS SERIES DECODER DATA GUIDE

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REMOTE CONTROL OVERVIEW

Wireless remote control is growing in popularity and finding its way into more

unique applications. Remote Keyless Entry (RKE) systems for unlocking cars or

opening garage doors quickly come to mind, but how about a trash container that

signals the maintenance office when it needs to be emptied? The idea behind

remote control is simple: a button press or contact closure on one end causes

some action to be taken at the other. Implementation of the wireless RF stage

has traditionally been complicated, but with the advent of simpler discrete

solutions and modular products, such as those from Linx, implementation has

become significantly easier.

Encoder and decoder ICs are

generally employed to maintain the

security and uniqueness of a wireless

RF or IR link. These devices encode

the status of inputs, usually button or

contact closures, into a data stream

suitable for wireless transmission.

Upon successful recovery and

validation, the decoder’s outputs are

set to replicate the states of the

encoder’s inputs. These outputs can

then be used to control the circuitry

required by the application.

VCC

ENC Tx

HS

LR

Series Series

Rx DEC

LR

HS

Series Series

GND

Figure 5: Remote Control Block Diagram

Prior to the arrival of the Linx HS Series, encoders and decoders typically fell into

one of two categories. First were older generation, low-security devices that

transmitted a fixed address code, usually set manually with a DIP switch. These

products were easy to use, but had significant security vulnerabilities. Since they

sent the same code in every transmission, they were subject to code grabbing.

This is where an attacker records the transmission from an authorized

transmitter and then replays the transmission to gain access to the system.

Since the same code was transmitted every time, the decoder had no way to

validate the transmission.

These concerns resulted in the development of a second type of encoder and

decoder that focused on security and utilized a changing code to guard against

code grabbing. Typically, the contents of each transmission changes based on

complex mathematical algorithms to prevent someone from reusing a

transmission. These devices gained rapid popularity due to their security and the

elimination of manual switches; however, they imposed some limitations of their

own. Such devices typically offer a limited number of inputs, the transmitter and

receiver can become desynchronized, and creating relationships and

associations among groups of transmitters and receivers is difficult.

The HS Series offers the best of all worlds. The HS Series uses an advanced

high security encryption algorithm called CipherLinx™ that will never become

desynchronized or send the same packet twice. It is easily configured without

production programming and allows for “button level” permissions and unique

encoder and decoder relationships. Eight inputs are available, allowing a large

number of buttons or contacts to be connected.

To learn more about different encoder and decoder methodologies please refer

to Application Note AN-00310.

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HS SERIES OVERVIEW

The HS Series encoder encodes the status of up to eight buttons or contacts into

highly secure encrypted serial data stream intended for wireless transmission via

an RF or infrared link. The series uses CipherLinx™ technology, which is based

on the Skipjack algorithm developed by the United States National Security

Agency (NSA). The CipherLinx™ protocol in the HS Series has been

independently evaluated by Independent Security Evaluators (ISE). A full

evaluation white paper is available at www.linxtechnologies.com/cipherlinx.

The encoder combines eight bits representing the state of the eight data lines

with counter bits and integrity bits to form a 128-bit message. To prevent

unauthorized access this message is encrypted with CipherLinx™ in a mode of

operation that provides data integrity as well as secrecy. CipherLinx™ never

sends or accepts the same data twice, never loses sync, and changes codes on

every packet, not just every button press.

Decoding of the received data signal is accomplished by the HS Series decoder.

When the decoder receives a valid command from an encoder, it will activate its

logic-level outputs, which can be used to activate external circuitry. The encoder

will send data continuously as long as the SEND line is held high. Each time the

algorithm is executed, the counter is decremented, causing the code to be

changed with the transmission of each packet. This, combined with the large

counter value and the timing associated with the protocol, ensures that the same

transmission is never sent twice.

An 80-bit key used to encrypt the data is created in the decoder by the user. The

decoder is placed into Create Key Mode, and a line is toggled 10 times, usually

by a button. This is required to gather entropy to ensure that the key is random

and chosen from all 280 possible keys. A high-speed timer is triggered by each

rise and fall of voltage, recording the time that the line is high and low. The 80-

bit key is generated by combining the low-order bits of the twenty timer values.

To create an association, the key, a 40-bit counter, and a decoder-generated ID

are sent to the encoder via a wire, contacts, IR, or other secure serial connection.

The HS Series allows the end user or manufacturer to create associations

between the encoder and decoder. If the encoder and decoder have been

associated through a successful key exchange, then the decoder will respond to

the encoder’s commands based on its permissions. If an encoder has not been

associated with a decoder, its commands will not be recognized.

The user or manufacturer may also set “button level” permissions. Permission

settings control how the decoder will respond to the reception of a valid

command, either allowing the activation of an individual data line or not. The

decoder is programmed with the permission settings during set-up, and those

permissions are retained in the decoder’s non-volatile memory.

The HS decoder has the ability to identify and output a decoder-assigned identi-

fication number for a specific encoder. An encoder’s key, a 40-bit counter, and

permissions are stored in one of fifteen memory locations within the decoder.

The decoder is able to output an 8-bit binary number that corresponds to the

memory location of the encoder’s information. This provides the ability to identify

the specific encoder from which a signal originated. This identification can be

used in various ways, including systems that record access attempts or in

applications where the originating user needs to be known.

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