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Baseband Processor for FRS, MURS, PMR446 and GMRS 'Leisure Radios'



  • Automatic Signal Scanning and IRQ on Detection of Valid Rx Signals
  • XTCSS Channel and Data Signalling
  • Tone Generation for Caller Recognition Tunes
  • Low Power Operation with Graduated Powersave
  • Programmable Signal Detection Thresholds
  • Aux ADC Available to Monitor Inputs
  • Quiet-Operation by Removing Unwanted Calls
  • Selectable Voice Companding
  • Voice Processing Facilities:
    • Tx and Rx Path Gain Setting
    • Voiceband and Sub-Audio Filtering
    Programmable Soft Limiter
  • C-BUS Serial Host Interface
  • Tx Interface for Single or Two-Point Modulation
  • Enhanced CTCSS and DCS (23/24-bit) Codecs All-Code and Monitor-Mode for CTCSS
  • Robust Automatic Packet Data Protocol with:
    • Error Correction and Detection
    • Interleaving and Data Scrambling/Privacy


  • FRS, MURS, PMR446 and GMRS 'Leisure' Radios

Supply Requirement:

  • 2.7 to 3.6 V

The CMX883, a full-function half-duplex audio and signalling baseband processor IC for FRS and PMR446 type facilities for both complex and simple end-designs.
Under the control of the host µC, all voiceband requirements are catered for:
Voiceband and sub-audio filtering, pre/de-emphasis, compression and expansion and audio routing and global level setting with single or two-point modulation in the transmit path.

The combination of new and standard signalling functions of this product offer, under software control, increased functionality, versatility and privacy.
Standard Extended-Code CTCSS and DCS is integrated with the new XTCSS code implementation. XTCSS provides more and better squelch-centred privacy codes with the added advantage of ‘silent operation’; no annoying interference from other sub-audio users.
XTCSS fitted radios enjoy more privacy and flexibility of operation. The CTCSS configuration of this product enables ‘Tone Cloning’.

Additionally two unique features of this product are its CTCSS ‘All Call Code’ and ‘All Codes Monitor’ modes:

All Call Code – transmissions using this code will be heard by all CMX883 enhanced radios regardless of their selected CTCSS code. This provides an important benefit to both safety and convenience.

All Codes Monitor – selection of this code at the receiver enables all transmissions that are using a CTCSS tone to be heard, and the tone number to be reported. Open channel noise or calls lacking coding, will go unheard. This is a superior method of ‘channel monitoring’, which allows miscoded calls from conventional CTCSS-party radios to be heard and directly responded to.

XTCSS Signalling - A state-of-the art (squelch) signalling format, employing both sub-audio (CTCSS) and in-band (XTC) signalling concurrently, which offers more than twice as many privacy codes as standard CTCSS operation and completely eliminates interference caused by other traffic on the channel (quiet operation).
Additionally the XTCSS signalling can be employed as on over-air control for such features as voice-scrambling and voice-compression.
XTCSS is fully compatible with both conventional and enhanced CTCSS signalling operations and will implement the ‘All Call Code’ and ‘All Codes Monitor’ functions.

With ultra low power requirements and graduated powersave, this product only requires a smaller, lower-power µC than existing FRS/PMR446 solutions, is available in compact SSOP and TSSOP packages.

Block Diagram


PMR Public Mobile Radio System. Also referred to as Land Mobile Radio (LMR) and/or Public Land Mobile Radio (PLMR).
PMR is generally a term covering peer-to-peer and repeater type communications.
MURS Multi Use Radio Service:
MURS, which is usually license-free, permits the use of a variety of emission modes in local (short-range) areas, but the most common mode is analoge voice using FM modulation. Data communications are permitted.
FRS Family Radio Service: A short range, low-power license-free FM radio system. FRS emissions are limited to 500mW (0.5W 2011). May operate using voice, data and signalling.
GMRS General Mobile Radio Service:
A (US) licensed land-mobile style FM UHF radio service. GMRS is considered a short range (line-of-sight) system with channels arranged in pairs for simplex and/or repeater working. May operate using voice, data and signalling.
This system is common in Europe and many European countries use a similar 8 channel system near 446MHz known as PMR446

Design Support Information

CMX882 Pseudo Code Implementation The CMX882 is a sophisticated modem + baseband processor targeted at “family radio” applications (e.g. FRS/GMRS/MURS/PMR446 radios). It is particularly well suited to family radios that include facilities for text messaging and GPS location data exchange. The purpose of this application note is to illustrate how the CMX882 can be configured for a feature-rich “family radio” product.
HSC and XTCSS Coding for Radio and Paging Systems The Hexadecimal Sequential Code (HSC) is a coding format for sequential tone signalling systems. HSC permits address codes, instruction codes and data blocks to be exchanged on an unrestricted basis between all units in a network, thus providing the key to the design of fully integrated base/mobile/personal communications schemes. HSC is also compatible with the frequencies and protocols used for international '5-tone' selective call (SelCall).
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