May 2006, Issue 190

ARM-Based Modern Answering Machine
Philips ARM Design Contest 2005 First Prize

VISUAL SIMULATION

One of my goals for this project was to experiment with the implementation of DSP features in a generic RISC processor. The starting point is a sample interrupt, which is actually a timer interrupt in this case. If higher accuracy and low jitter are required, the interrupt should actually come from the ADC, and the ADC value should be buffered until it’s read by the interrupt routine. But this feature isn’t available on the LPC2138. The same timer interrupt is also used to output samples for voice playback and tone generation.

The DSP functions are often tricky and difficult to debug. Professionals use tools like MATLAB to develop and tune their algorithms. Amateurs generally don’t have access to such simulation tools. I tried a few different options.

One problem with demodulation is that you might be trying to catch a signal that shows itself and disappears quickly. On the other hand, a simulator allows you to generate millions of these furtive signals and verify that the algorithm works well in the presence of noise or any other hindrance. I developed and debugged the DTMF demodulator using an old version of Microsoft Visual C/C++. Although I couldn’t use that version to develop modern applications to run on Windows XP, it was an excellent tool for developing generic C code. It also has a step function that can be used to track down nasty bugs. This was a nice feature because I didn’t have a step function on my LPC2138 platform.

To debug the DTMF detector, I generated some DTMF samples and then sent them to my demodulation routine. Until I got the routine to correctly extract the right DTMF tones, I invested my time on the Windows platform rather than the ARM-based platform. After the DTMF detector was debugged in Windows, it worked the first time it was ported to the LPC2138! You may download the Visual C/C++ source code from the Circuit Cellar FTP site.

Because generic RISC processors like the LPC2138 don’t have a saturation function, it’s also important to ensure that the input values or the size of the variable are correctly scaled to prevent saturation. Refer to the “DP Versus RISC” sidebar for more information on the subject.

I started developing and debugging the FSK demodulator in an Excel spreadsheet. Excel isn’t necessarily a perfect DSP development tool, but it has a complete set of arithmetic functions and graphics tools.