32-bit MCU for a true single-chip digital audio decoder

With the market for digital audio exploding in recent years, a wide number of chips or chipsets have been introduced to meet the demands of ever better players. But there are many pitfalls for those entering into the digital audio market. And it’s often a question of selecting the proper processing hardware.

The standalone audio and multimedia player has dominated the digital audio market in recent years, and millions of consumers are trying to figure out how to best connect their portable player to their home and car stereos. This has prompted the established vendors of home and car audio to prepare their Hi-Fi systems for the digital age.

Adopting computer hardware
Some vendors have tried to adopt computer components and apply them into audio or multimedia players. But while excelling at their intended task of handling vast amounts of data in a multi-tasking computer, these components present some big challenges to the audio engineer because of their relatively slow and unpredictable task switching and poor real time performance. Such systems suffer from high power consumption, and demand significant overhead in CPU speed before uninterrupted playback can be guaranteed. They also suffer from their poor level of integration; most systems still require three or four chips, including the MPU, SDRAM, NAND flash and audio codec, and further reduction is not yet possible because modern wafer processing technology doesn’t allow these technologies to be mixed on the same die.

The ASIC approach
Other vendors have adopted single-chip ASICs. Because the traditional microcontroller units (MCUs) offered only a fraction of the processing power required to decode the digital audio content, a custom MP3 or similar decoder was added to existing microcontrollers. This solution has long been viewed as preferable by portable audio player manufacturers because the single IC meant a smaller board could be designed.

Another advantage of this solution was the reduced power consumption, which allowed much smaller batteries to be used. The low power consumption is in part made possible because the DSPs required a lower system clock speed, and also because the external memory busses were eliminated. But while preferred for size and power consumption, this approach has proven to be a very risky bet because the appearance of new standards and rapid shifts in consumer expectations meant that a new product could be outdated even before the first product was ready to ship to stores.

The perfect audio decoder
Obviously, the ideal solution would be an off-the-shelf microcontroller with a CPU powerful enough to decode the audio content without a custom built audio decoder. This would offer all the advantages of the single chip ASIC, but the lack of hardware that could get outdated would also guarantee the platform would be flexible enough to accommodate the unpredictable shifts in customer demand.

Sure enough, where there is demand there is usually a solution. Atmel’s latest 32-bit AVR microcontroller, the AT32UC3A3, offers the required DSP performance, and features very innovative DMA solutions. This translates into a highly predictable audio decoder capable of achieving high levels of audio quality. Atmel has also seen the business opportunity in the digital audio sector, and has designed a complete reference design which requires minimal modifications before it can be turned into a production item.


Atmel’s AT32UC3A3 32-bit AVR MCU offers the required DSP performance, and features very innovative DMA solutions.

The brains of the 32-bit AVR UC3 MCUs lie in the UC3 CPU core, which offers a wide range of DSP instructions usually only found in high-end CPUs and DSPs. The MCU eliminates the need for custom audio decoder hardware, and it is capable of decoding a stereo MP3 stream while strolling along just above 20MHz. With a maximum speed of up to 72MHz, this means there is plenty of CPU performance left to handle “heavier” audio formats such as AAC and AAC+.

The 32-bit AVR UC3 microcontroller offers a Hi-Fi stereo 16-bit bit stream DAC. This requires only a small external power amplifier to generate the output voltages required for line, headphones or external speakers. Playback in four channel audio or full surround sound requires an external audio codec, which much be connected to the microcontroller’s IIS interface.

While the 32-bit AVR UC3 microcontroller has enough flash and SRAM to store the firmware, decode the audio, and buffer the communication, there is not enough on-chip memory to buffer more than a couple of seconds worth of audio content. But the UC3 offers a wide selection of memory storage options, the three most popular being SD/MMC cards, USB mass storage, or NAND flash. A device can use any combination of these to store the audio content.

Another important feature for digital audio devices is the speed at which music and other digital content can be transferred in and out of the system. Streaming a single digital audio track may require a bandwidth of just 200kbps or less. But for applications with mass storage of an audio library, the consumer demands much faster communication for fast synchronization of a large music library. For this purpose, the AT32UC3A3 device has a Hi-Speed USB interface, and an MMC/SD port which supports WLAN over SDIO. Other family members such as the AT32UC3A0 and AT32UC3A1 devices also offer a Full speed USB, and a 100Mbps Ethernet port.

The many high speed communication interfaces offered by the 32-bit AVR UC3 microcontrollers means a single SRAM would quickly become the bottleneck of the system. Predicting this, Atmel has added no less than four SRAMs to the AT32UC3A3. Two of these even offer dual port access to further speed up communication and avoid collisions. This ensures that the SRAM bandwidth is never limiting transfer speeds throughout the system. And most importantly, it guarantees that audio playback quality is never compromised when the high speed even when high speed communication is running in the background.

The backbone of the UC3 microcontrollers is the multi-layer high speed bus. This allows the CPU and peripherals to share much more data every cycle by allowing simultaneous access between multiple masters and slaves.


Atmel's AVR UC3 MCU block diagram.

Atmel is one of the world’s leaders in low power MCU design, and the 32-bit AVR UC3 is no exception. The power consumption is less than 2mW/MHz, allowing the UC3 microcontroller to deliver more than 150 hours of audio playback from two AA batteries. In standby mode with only Real Time Clock running, a device can stay in the drawer for more than 9 years.

Reference design with software
To demonstrate the digital audio capabilities of the 32-bit AVR UC3 microcontrollers and reduce the time to market for potential customers, Atmel has designed two reference designs named EVK1104 (AT32UC3A3) and EVK1105 (AT32UC3A0). The kits demonstrate playback from USB mass storage device, SD card or NAND flash, and also shows how music can be downloaded or streamed from a PC through the USB interface. An upgrade kit is already planned which will add WLAN, Bluetooth, and Zigbee RF connectivity to the kits.

Atmel