The Cortex M0+ microcontroller from ARM is well-suited for audio decoding applications in USB memory sticks due to its low power consumption, small size, and built-in DSP capabilities. By leveraging the Cortex M0+ processor, USB drive manufacturers can add high-quality audio playback to standard thumb drives without significantly increasing cost or power draw.
Overview of the Cortex M0+
The Cortex M0+ is a 32-bit ARM processor optimized for low-power embedded applications. Key features include:
- 32-bit ARM Cortex-M0+ core running up to 50 MHz
- As low as 9.2 μA/MHz in active mode and 5.5 μA in sleep mode
- Integrated digital signal processing (DSP) engine
- Memory protection unit for enhanced security
- Single-cycle I/O port and interrupt handling
- Thumb-2 instruction set for improved code density
This combination of small size, low power draw, and DSP performance makes the Cortex M0+ well-suited for audio playback applications. The availability of a wide range of M0+ microcontrollers from vendors like STMicroelectronics, NXP, Microchip, and others allows manufacturers to easily integrate the Cortex M0+ into their USB drive designs.
Decoding Audio with the Cortex M0+ DSP
A key advantage of the Cortex M0+ is its integrated DSP engine, which is designed to efficiently process digital signals. This DSP can be leveraged to decode compressed audio files without overburdening the CPU.
Common audio compression algorithms like MP3, AAC, Ogg Vorbis, WMA, and others can be decoded in real-time by the DSP engine. The DSP provides features like a multiply-accumulate unit and flexible addressing modes to enable these complex decoding tasks.
Because the DSP is integrated directly into the microcontroller, no external components are required. The M0+ manages all the DSP processing, input/output, file management, and control functions needed for audio playback on a USB drive.
Interfacing External DAC and Amplifiers
To output the decoded audio, an external digital-to-analog converter (DAC) chip and amplifier circuit can be interfaced to the M0+ microcontroller. The microcontroller sends the decoded digital audio stream to the DAC which then converts it to an analog voltage for the amplifier.
The DAC and amplifier selection depends on the audio quality and loudness requirements. High-fidelity playback can be achieved by using a 24-bit DAC and quality amplifier IC. Lower-cost 8 or 16 bit DACs may be sufficient for applications like voice playback.
I2C, SPI, or dedicated audio buses like I2S can be used to interface the external DAC and amp. Microcontrollers like the STM32 M0+ series have dedicated I2S interfaces that help reduce CPU overhead during audio playback.
Implementing the USB Interface
A USB controller handling Full Speed USB 2.0 is integrated into the M0+ to allow easy connection as a thumb drive. No external USB device controller is required, reducing BOM cost.
The M0+ USB interface can provide the digitized audio decoding along with standard flash drive functions like file storage and transfers. From the PC host’s perspective, an M0+ USB drive with audio playback looks identical to a standard USB mass storage device.
Vendor-specific application software can be utilized on the host PC side to control audio playback on the drive. This user-level software sends commands via USB control transfers to tell the drive when to start, stop, pause audio, adjust volume, etc.
Optimizing for Low Power
A key consideration in battery-powered USB drive designs is minimizing power consumption during audio playback. The Cortex M0+ is well-suited for this, with very low active and sleep current draw.
To optimize power, the M0+ can be run at reduced speeds when not actively decoding audio. When no playback is occurring, the processor can be put in sleep mode, only waking periodically to check for playback requests from the PC.
The DSP and memory can also be powered down when not needed to further decrease power use. With careful design, the average power draw can be kept to just a few milliwatts during playback, comparable to an Bluetooth headset.
Example Implementations
There are several examples of Cortex M0+ microcontrollers being leveraged to enable audio playback on USB drives:
- STMicroelectronics offers their STM32 M0+ Family Discovery Kit for Audio which demonstrates USB audio streaming using their M0+ chips.
- Microchip’s SAM L10/L11 microcontrollers combine Cortex M0+ cores with USB and audio interfaces targeted at portable audio applications.
- NXP’s LPC800 M0+ series has been used by some aftermarket car stereo vendors to add USB audio capabilities.
- The Cypress FX2LP microcontroller incorporates both a Cortex M0+ and a USB 2.0 interface, intended for audio accessory applications.
These and other M0+ microcontrollers can provide OEMs with tested reference designs to help shorten their development time for Cortex M0+ based USB audio products.
Conclusion
The Cortex M0+ provides an ideal foundation for adding high-quality audio playback to USB thumb drives. With its integrated DSP and low power consumption, combined with wide microcontroller availability, manufacturers have all the key ingredients needed to develop compelling USB audio solutions.
As demand grows for more multifunctional USB drives, the Cortex M0+ stands ready to enable next-generation thumb drive designs with robust support for digital audio decoding and playback features alongside standard data storage.
