Product description

The EV6550DHAT brings together the Raspberry Pi and the CMX655D Ultra-low Power Voice Codec in a convenient and inexpensive way, enabling easy demonstration of the CMX655D and providing improved low power audio capabilities to the Raspberry Pi. This is achieved using standardised hardware, opensource software and native drivers, creating a platform that is easy to use and that simplifies product development.

One of the major benefits of the hugely-popular Raspberry Pi is its low power, flexibility and accessible GPIOs which encourage developers to expand on its basic functionality. The multi-way GPIO header on the latest versions of the board consists of 40 mixed-function pins, 26 of which support general purpose input/output connections.

The Raspberry Pi HAT (Hardware Attached on Top) is a hardware standard defining an add-on board that mounts directly onto the Raspberry Pi and attaches to the GPIO header. The EV6550DHAT conforms to this standard and integrates the functionality of the CMX655D Ultra-low Power Voice Codec into the Raspberry Pi environment.

The EV6550DHAT enables the CMX655D’s feature set (two matched channels supporting a wide variety of digital MEMS microphones, signal processing and a 1W high-efficiency Class D speaker amplifier) to be easily demonstrated. The most immediate benefit is an improvement to the standard low power audio capabilities of the Raspberry Pi.

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  • Two top-ported digital MEMS mics
  • Easy screw connection for external speaker
  • Flexibility to use local or off-board power for the PA
  • Accessible GPIO and test pads to allow further options (e.g. external MEMS)
  • User-modifiable prototype space
  • Option to add line out phono connector
  • Open-sourced ALSA compatible driver


  • Demonstrates CMX655D functionality
  • Product development with the Raspberry Pi

Power supply requirement

  • 5V sourced from the Raspberry PI GPIO pins

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Design resources

EV6550DHAT High Resolution Schematics. Version A04.

EV6550DHAT High Resolution PCB Overlay. Revision A

EV6550DHAT Gerbers. Revision A

A simple start guide to help users get started evaluating the EV6550DHAT. Version 1

EV6550DHAT Bill of Materials (BoM). Revision A Mod 4.

EV6550DHAT Raspberry Pi GUI API Reference

EV6550DHAT Raspberry Pi GUI and source code. Version 1.0 – GUI and source code supplied as three split 7-zip compressed files ready for download. Once all three files are downloaded they must be renamed from EV6550DHAT_00x.7z to EV6550DHAT.7z.00x and then decompressed and recombined using 7-zip to access the GUI and source files.

Download 1

Download 2

Download 3

EV6550DHAT Raspberry Pi ALSA Driver User Guide and Reference

EV6550DHAT Raspberry Pi ALSA Driver Installer

Q: Why when I look at the CMX655D PA output with an oscilloscope I do not see the expected analogue audio waveform?

A: Assuming that the CMX655D overload current and thermal protection circuits have not been triggered then the signal measured across the AOUTN and AOUTP pins will not be a true analogue representation of the sampled audio. This is because the PA is a Class D design where the audio is represented by a modulated PWM signal and as with all Class D amplifiers additional filtering and loading of the output is necessary for correct operation. Typical methods of measurement use high impedance probes or signal inputs and so do not appropriately load the amplifier.  Therefore, to achieve a meaningful measurement of the sampled audio the following intermediate output components are advised.




Q: The CMX655D Ultra Low Powered Audio Codec with PA, supports 8, 16, 32 and 48ksps sampling rates, however my application requires a sampling rate of 44.1ksps, a common rate used in recording studios and for CD’s.  Should I consider the CMX655D?

A: The CMX655D Ultra Low power codec IC was developed to support portable and low powered applications such as handheld radios and intercoms systems, but it has been found useful in many other applications.

The CMX655D is a fully digital codec and as such all internal functions such as the filters and the sampling rates are derived from an internal 24.576 MHz clock.  For example, 24,576,000 / 512 = 48 kHz, one of the default sampling rates.  Therefore, it is possible to support 44.1 kHz by scaling this clock proportionately, for example while 48 kHz is set: 22,579,200 / 512 = 44.1 kHz

The internal 24.576 MHz clock may be sourced from the RCLK pin or from the integrated PLL.  A recent test was performed to demonstrate that 44.1 kHz operation using a clock sourced from the Serial Audio Interface (SAI) was possible.  To achieve this the CMX655D was operated as a clock slave with a 48 kHz sample clock (where the integrated PLL is set to be referenced to the sample period «LRCLK/FS»).  Then by sending audio samples via the SAI with a sample rate of 44.1 kHz, the audio could be reproduced accurately at the Class D, PA output.

It should be noted that this method may work with any sample rate from 32 kHz up to 48 kHz. All internal clocks will scale and will affect both the microphone and speaker audio paths.  However, it should be noted that the CMX655D would be operating outside the datasheet stated limits, so caution is required if attempting a new untested clock.

Note: The EV6550DHAT does not currently support sample rates outside those quoted in the CMX655D datasheet although possible with additional modification.

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