Introduction to code-hopping products

Code-hopped messages are ideal for use in simple low-cost one-way remote control applications, especially where security is required.

No knowledge of encoding or modulation is required to use these versatile new modules and products from Radiometrix.

Typical applications include:

  • Remote locking
  • Burglar alarms
  • Door entry systems
  • Industrial remote control
  • What is RF code-hopping technology?

Code-hopping means that code contained in a transmitted message changes or ‘hops’ with every new transmission. If a transmission is sent containing for example, a control signal and then at some time later the same control message is repeated, the encoding of the second message will be different from the first, even though the control being sent is the same.

  • Why is this useful?

Anyone equipped with a suitable receiver can monitor and record transmissions. If the remote control in question were being used to disarm a burglar alarm system it would be unwise to use a standard fixed-code message to do this because the message could be recorded and later re-transmitted by a would-be thief, allowing them to disarm the alarm system. Some older car central-locking systems with RF ‘key-fob’ control facility were vulnerable to this kind of “code-grabbing” activity and therefore an ever changing or hopping code was developed for this type of application.

  • Why not just encrypt the message?

In a security orientated remote control application, correct identification of the sender is required. Preventing the “identity fraud” of the sender is the aim of the code-hopping transmitter. Encryption by itself is not useful, since encrypted signals can be recorded and re-transmitted in the same manner as non-encrypted ones. The encryption process must cause the transmitted code to change for every message, even if the message being sent is exactly the same each time, thus preserving the uniqueness of the legitimate sender.

Radiometrix code-hopping modules and products can be viewed as a means of achieving ‘secure authentication’, since each transmitter is unique and must be synchronised to the receiver-decoder being used in the remote control application before controls will be processed. The aim of the technology is to provide secure RF remote controls, rather than to encrypt any control data contained within the messages.

  • What is the “hop-code” made of?

The core of the code-hopped message is a 32-bit hop-code that changes with every new transmission. This hop-code is encrypted by means of a 64-bit key, combined with a 28-bit unique I.D. (serial number) and a 4-bit control or function Code. The 64-bit key is invisible to the user and is never sent over the RF link.

The 32-bit hop-code itself contains a 16-bit synchronisation value, a 12-bit discrimination value and a copy of the 4-bit function code. The 16-bit synchronisation word is used as a counter for hop-code transmissions whilst the discrimination value does not change and is utilised as part of the decryption process.

  • How easy is it to use this technology?


In the case of our code-hopping transmitter module, KTX2, simply connect a suitable power supply and antenna. Connecting the module function pins (individually or in combination) to the power supply will cause the module to send code-hopping transmissions. Push-buttons may be connected directly to the function pins if required. There are four function pins providing fifteen individual control messages from the module. Each module is pre-programmed with its own unique serial number and is ready for use.


Even easier to use is the KFX2 key-fob type transmitter with integral antenna. Simply press one of the five buttons on this battery powered transmitter to send a code-hopped message.


Stand alone Receiver-Decoder RF module

New from Radiometix is the KRX2 Receiver-decoder module. This compact high performance 433MHz FSK RF module is optimised for use with the built-in code-hopping
decoder, compatible with Radiometrix code-hopped messages. Its digital outputs may be connected directly to LEDs or other discrete components and it is the fastest and simplest way to integrate secure wireless remote control into an exisiting product.


Complete Receiver + Decoder

To receive, decode and make use of the control messages is easy with KDEC:

These boards are pre-programmed to process messages from Radiometrix code-hopping transmitters such KTX2, KTX1 and KFX2 and include a high-performance Radiometrix UHF or VHF FSK receiver module.

The board has relays and screw terminations so that you can realise your own secure control application immediately.

There are momentary and latching modes and user-select KTX or KFX optimisation modes. Press the “Learn” button, initiate a transmission from your KFX2 or KTX module and the “Learn” indicator will flash to show that the KDEC will now act upon controls received from your transmitter.

That’s all there is to it!


Stand alone Decoder

If you require the flexibility of your own PCB layout, we offer the KRX416 decoder I.C., which you can easily interface to RF modules, microcontrollers and other discrete components as you wish.

Technical Brief – code-hopping modules and products

Radio Frequency
 433.92MHz (VHF frequency also available)
Modulation FSK
Typical range Up to 300m (433MHz), antenna dependent, obstacle free path
Data encoding Manchester, Code-hopping (32-bit)
Data rate 1250bps approx.
Single message length 67 bits
Minimum transmission 150ms approx.
Error detection CRC is applied to the raw message and an additional control function code check occurs during decryption to eliminate the possibility of an incorrect output from the decoder
Number of functions KTX2 module: 15 (or 4 in parallel, from the 4-bit function code).
KFX2: up to 5 buttons
Other data included within the message
Serial number (28-bit),
Supply voltage status (1-bit) – for transmitter “low battery” warning on KDEC and KRX2

For further technical information, please refer to individual datasheets.