Design Resources


 Q. I am considering migrating from the CMX813 to the CMX823, to take advantage of it programmable tone table, can you help me understand the differences between these two devices

A. The CMX823 should be considered to be an enhanced CMX813, in that it is pin, package and virtually function compatible. The CMX823 takes the excellent CMX813 and adds full tone programmability so that user tone sets are now possible, enhanced S/N tone detection performance and improved tone resolution and accuracy. In terms of hardware it is a plug and play solution.

In Software there are two additional registers to be made aware of and some differences in existing Registers, the following table identifies which registers differ. For more detail on the register specifics please refer to the respective data sheets.

 Write Registers
  Register Names CMX813 CMX823
$01 General Reset Compatible
$30 Control Not Compatible
$31 Tone Address OK N/A
$32 Auxiliary Control N/A OK
$33 General Control N/A OK
 Read Registers
$38 Decoded Tone Address Not Compatible
$3F Status Not Compatible

Q. I need a replacement for the xX307. What should I use?

A. The xX307 was a receiver IC that utilized external components to program the center frequencies and detection bandwidths of three sequential tones from 200Hz-3000Hz. While there is no pin-compatible replacement for the xX307, the CMX823 is the most cost-effective device that can perform the functions of the xX307. Please be aware that there are many differences between these two devices, most notable being the fact that the CMX823 requires a microcontroller while the xX307 did not.

Q. Do you have a calculator available to work out the hex values to be programmed into the Tone Parameters Register ($34)?

A. Yes – There is an Excel spreadsheet available to calculate values for the Tone Parameters Register ($34) in the CMX823 Tone Parameters Register Calculator [updated November 2007].
Note: The Excel Add-in, ‘Analysis ToolPak’ must be loaded to use the spreadsheet. Please see the Excel help for help on loading Add-Ins.

Q. I have noticed that in some of the 2 tone or 5/6 tone decode sequences the TONE CHANGE bit ($3F b5) is not set between the first and second received tones, do you have any advice?

A. It is important to appreciate the note in section 1.5 of the CMX823 datasheet that states:
RAM location 0 is reserved for programming the 1st tone of a 2-tone or 5/6 tone sequence.
This is intended to stop the possibility that a second tone is written to the first location, this may in some circumstances cause the failure of the TONE CHANGE bit being set correctly. It is sensible to avoid writing a valid tone to the first RAM location 0 if at all possible.

Another recommendation for users who receive concatenated 2 or 5/6 tone sequences is to set the CMX823 to a NOTONE state between success sequences. This will avoid possible false readings of the TONE CHANGE bit.

C-BUS Guides

Guides to operation and interfacing


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