One of my personal goals for this conference was to become better acquainted with the various wireless technologies available on embedded platforms. As mobile devices (cell phones and tablet computers, in particular) become more equipped with wireless protocols (Bluetooth, Wi-Fi, NFC, etc.), more and more products with these same technologies are reaching the market. On top of that, hobbyists and open source developers have created some incredibly cool wireless-enabled widgets.

Near-Field Communication (NFC) is a fairly new wireless technology that is becoming more and more prevalent as mobile device manufacturers start to include the necessary hardware on their products. One of the Technology Day morning sessions focused specifically on this protocol as it relates to the embedded world.

In a nutshell, NFC is a short-range 13.56MHz wireless technology that supports peer-to-peer communication in addition to reading passive “tags.” It is very much an extension of the familiar RFID technology that has become a valuable asset in physical access control, contactless payment, logistics, and many other applications, and NFC’s two-way capabilities add to the list of potential uses.

One of the coolest things I saw during the seminar was assisted Bluetooth pairing using NFC. Just about anyone who has tried to pair their mobile phone or computer to a Bluetooth-enabled device will tell you that the process is a huge pain. For example, I was recently on vacation with a few friends and driving in a rental car. The car had a Bluetooth-ready entertainment console that could be used as a wireless speaker. Between the four of us, it took 3 phones, two computers, and about 20 minutes before we got a successful connection! Thankfully, it only took about 6 minutes for our blood pressures to return to normal.

Last year, TI released this handy app note to complement a MSP430-based development kit they released that demonstrates, among many other functions, NFC-assisted Bluetooth pairing. The operation is super-simple; the presenter was able to connect his Android phone to the device over Bluetooth by simply waving the phone next to the NFC antenna on the board. Within seconds, he was manipulating LEDs on the board with the TI-developed Android app. To prove that the connection wasn’t just NFC, he asked a volunteer to take the phone, move outside of NFC range (a few feet was enough in this case), and play around with the LEDs. As you can imagine, it worked like a charm. This is going to make me even more frustrated the next time I try to pair two Bluetooth devices without NFC…

The Internet of Things is a concept that has been rattling around in the brains of tech junkies and visionaries for quite some time now. The idea is simple: make everything accessible and controllable using the Web. Your refrigerator, for example, can automatically order new groceries when its stock is running low. Your car might send you automatic email notifications when it’s getting close to oil change time. The possibilities are vast. So, however, are the technical challenges of getting such a network established.

A key component of this effort is deciding on a wireless technology. A huge number of households across the globe have access to some sort of Wi-Fi network, whether it be a router at home or free public access. This fact alone makes Wi-Fi a viable choice. TI, as well as several other silicon IC manufacturers, has released Wi-Fi-enabled products to contribute to this rising market. One of the sessions at this year’s Technology Day covered one product in particular, the CC3000 module. The CC3000 handles a great deal of the Wi-Fi connection and communication tasks, which frees up the main processor to run the application software. Communication with the processor is over SPI, which makes it very portable to new or existing designs, regardless of the microprocessor manufacturer.

One example for a simple project that could use this Wi-Fi module is something I’ve had worked out in my head for a few months now. Like a handful of other DMC employees, I take public transit to work. As a result, I’m tethered to the Chicago bus schedule and a slave to the (oft unpredictable) Midwestern weather. What I’d like to do to stay prepared and on top of my schedule is create a web-enabled device that will grab the bus tracking data, the current weather, the date and time, and perhaps an inspirational quote, and display them on a scrolling LCD by my apartment door.

Prototyping a device like this would be simple with an Arduino or an SOC device like the Raspberry Pi. The problem is, those devices are either too expensive (the Arduino + Wi-Fi shield) or too souped-up (the Raspberry Pi) to justify using them for such a simple gizmo. That doubles if I ever make more than one. Thus, if I ever get serious about making something like this, a cheap microcontroller and an embedded Wi-Fi module (like the CC3000) might make a lot more sense.

Throughout the day, I was able to network with TI engineers and customers, which is, in my opinion, one of the strongest benefits of events like these. It’s always great to see that other folks in the industry are as enthusiastic about the cutting edge of technology as we are at DMC! I have no doubt that we’ll be back for future Texas Instruments Technology Days.

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Picking the test project
There are so many options for a first project: flashing LED, doomsday device, iSomething…

Hm, iSomething sounds like a cool idea. How about making a poor-guy MP3 player? Maybe downgraded to the extreme: one song, poor audio quality, no control buttons?

To make a digital audio player we need two major components: storage memory for digital audio and Digital-to-Analog Converter (DAC). Expectedly, such a simple microcontroller does not have a built-in DAC, so it sounds like I would need a separate chip. But wait…

Who needs a DAC for lousy quality audio?
There is an easy alternative to DAC – Pulse Width Modulation (PWM). The idea is to generate an audio signal with variable-width digital pulses, which should produce acceptable quality audio with high enough pulse frequency. Sure enough, TI has an application note to demonstrate exactly that (PWM for audio application). Since I decided not to worry about audio quality, I picked 8-bit for audio resolution (as a side note – it is supposed to be roughly equal to AM radio quality). The 8-bit resolution gave me 256 possible PWM widths (for example, value 10 means that output stays on for 10 cycles and after that turns off for 255-10=245 cycles).

Assuming that my TI chip runs at the max speed (16 MHz) my complete PWM cycle would be: 16,000,000Hz/256=62,500 Hz. Wow, at such high PWM frequency I should be able to generate sound with frequencies up to 62,500Hz/2=31,250 Hz (this calculation is based on Whittaker–Nyquist–Kotelnikov–Shannon sampling theorem). This also means that I have to store 31,250 bytes of data for 1 second of music. Where am I going to store my music?

External storage
MSP430G2231 only has 2KB of memory. Since I need 31,250 bytes to store 1 second of music, I could store the whole 0.06 seconds using internal storage. This is definitely not enough.

This just means that I need external storage. How about a SD/MMC Card? It is the perfect external storage for microcontrollers because it is inexpensive and supports microcontroller friendly SPI protocol. The Launchpad came with 2 chips: MSP430G2211 and MSP430G2231 the latest has built-in universal serial interface that supports SPI protocol, perfect! After checking the TI website, I found this application note to interface between MSP430 and SD Card. Now I need to decide how to store the music.

Audio format battle: mp3 vs. au
It would be really cool to play mp3 files on the Launchpad, but it’s not going to work. For example, the smallest Helix mp3 decoder needs ~20K of program memory, and my microcontroller has only 2K. Of course I could add an external mp3 decoder – but that’s not fun.

Since mp3 is out, I could just write audio as a stream of samples (bytes) and add simple header to tell how many samples are in my file, sample rate, etc. It would be nice to use a standard audio file format, however, so I chose arguably the simplest audio file format ever created: au format. Even though it is quite old, the format is still supported by many applications including my favorite (open source) audio editor – audacity.

Getting audio file to the SD card
Storing files to an SD card is also tricky. For multiple reasons I decided not to use a file system on the SD card, so I have to write an .au file to the card as a raw data. This can be done by using HDD imaging tools. I’ve had good experience with DiskImage, although other disk imaging software (such as dd) should work as well.

Hardware
There are only two external components to the Launchpad used: the SD card and the earbud headphones. I connected the SD card power to one of the Launchpad pins, so I could reset the card by toggling its power. The earbud headphones connected to the Launchpad PWM output pin through a current limiting resistor. “But where is your low-pass filter?” curious readers will ask. Well, yes, I should be using a low-pass filter to smooth-out PWM pulses. However, headphones work well enough as a low-pass filter due to their mechanical nature. So, cancel the filter and connect everything together!

SD card access time issue
After fixing a few software bugs I finally heard something in the headphones. It was sort-of music but with some strange clicking noise. Such a disappointment; I thought I was done!

It turns out that as the data, which was stored on the SD card in 512 byte blocks, sometimes took time to switch between these blocks, and every so often I was missing a few samples and it caused these clicks. I added a small (16 samples) FIFO buffer between the SD card and PWM. The buffer made its magic and I was enjoying AM quality music for a whole 15 minutes.

Conclusion
It’s totally possible to play acceptable quality audio using a very basic microcontroller. The sample rate can be pretty high (~31kHz in my case), allowing you to achieve much better audio quality, although PWM resolution (8 bit) is a limiting factor.

One of the options to drastically improve audio quality would be to use two independent 8 bit PWMs and sum up their signals. This would increase the audio resolution to 16-bits which should sound (almost) like a CD quality. However, MSP430G2231 has only one PWM channel so it is not an option here.

Video
Here is a video of the Launchpad playing (kind of) music, enjoy.

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