Silicon Labs' Precision32™ SiM3U1xx 32-bit MCUs based on the ARM Cortex-M3 processor have peripherals and features capable of meeting all of these requirements. These MCUs are uniquely suited to an unconventional Class D power amplifier application by directly driving the speaker using a 5 V, 150 mA source, 300 mA sink high-drive I/O. The only external components required on the outputs to drive audio from an SiM3U1xx MCU are an inductor, some capacitors and a ferrite bead per output channel.
The high-drive I/Os also have programmable current limiting, enabling up to 16 levels of volume control without the need for firmware to scale the audio data, saving time and code space. Since these I/O pins are on a separate voltage network from the rest of the device; they can also interface with large, external, high-gate capacitance MOSFETs that are at any voltage level up to 6 V without the need for external drivers.
Figure 2: SiM3U1xx Device Features in a Class D Audio Application
In addition to the high-drive I/O, the high-precision enhanced programmable counter array (EPCA) operates at double the peripheral clock speed, which effectively provides 10.4 ns resolution and reduces output distortion. This peripheral is highly configurable and automatically generates center-aligned synchronized PWM signals; all the firmware has to do is copy the 9-bit quantized data to the EPCA registers to implement the Class D PWM.
The SiM3U1xx devices also include a crystal-less USB transceiver compatible with the USB audio interface, two 250 ksps 12-bit successive-approximation-register analog-to-digital converters (SARADCs), and an I2S receiver to implement audio streaming from a PC, portable music player or a wide range of I2S-enabled audio devices. Numerous digital communications peripherals such as three serial peripheral interface (SPI) modules, a 16-bit external memory interface with programmable timing and up to 256 kB of flash enable audio data storage coupled with an on-chip capacitive sensing block enable the development of capacitive button and slider user interfaces.
Finally, the priority-decoded dual-crossbar architecture implemented in Precision32 MCUs enables developers to shift peripherals dynamically or during configuration to maximize usage of the device pins. This innovative crossbar architecture allows an application to pick and choose the desired peripherals from a feature-dense device without nearly as much hassle and conflicts as other MCUs on the market.
To support audio application development for 32-bit designs, Silicon Labs offers the cost-effective Utility Class D ToolStick development platform (priced at $35 USD) with full source code available that implements a demonstration Class D amplifier using the smallest 40-pin 6 mm x 6 mm package in the SiM3U1xx MCU family. This demonstration and evaluation tool implements four modes of operation: sampling data from a portable music player using the ADCs, USB audio streaming from a PC, playing pre-recorded data stored from on-chip flash using a common audio compression algorithm, and a voice recorder that stores data in on-chip flash using a common audio compression algorithm. The development board also provides access to the I2S receiver pins.
Figure 3: Utility Class D ToolStick Board Features
Complimentary IDE and AppBuilder tools for Precision32 MCU products allow fast and easy adjustments to the example source code. With these easy-to-use tools, it's a snap to add Class D audio to any 32-bit embedded product using the innovative SiM3U1xx MCU family. For more details about the Class D ToolStick development platform, please visit www.silabs.com/toolstickclassd.
About the author
Tabitha Parker serves as a senior applications engineer for Silicon Laboratories' Embedded-Mixed Signal group, focusing on the company's microcontroller products and technologies. Ms. Parker joined Silicon Laboratories in 2004 as an applications engineer. She holds a bachelor's degree in Computer Engineering from California Polytechnic State University at San Luis Obispo.