7.2 Lab 4: lab

Implementation

As this is your first experience with the C environment, you will have the option to add most of the required code in C or assembly.A C skeleton will provide access to input samples, output samples, and interrupt handling code. You will add code to transfer theinputs and outputs (in blocks at a time), apply a hamming window, compute the magnitude-squared spectrum, and include a trigger pulse.After the hamming window is created, either an assembly or C module that bit reverses the input and performs an FFT calculation is called.

As your spectrum analyzer works on a block of samples at a time, you will need to use interrupts to pause your processingwhile samples are transferred from/to the CODEC (A/D and D/A) buffer. Fortunately, the interrupt handling routines havebeen written for you in a C shell program available at v:\ece420\55x\lab4\main.c . For this lab, you will be working with the code available at v:\ece420\55x\lab4 .

Interrupt basics

Interrupts are an essential part of the operation of any microprocessor. They are particularly important in embeddedapplications where DSPs are often used. Hardware interrupts provide a way for interacting with external devices whilethe processor executes code. For example, in a key entry system, a key press would generate a hardware interrupt.The system code would then jump to a specified location in program memory where a routine could process the key input.Interrupts provide an alternative to polling. Instead of checking for key presses at a predetermined rate (requires aclock), the system could be busy executing other code. On the TI-C55x DSP, interrupts provide a convenient way totransfer blocks of data to/from the CODEC in a timely fashion.

Interrupt handling

The main.c , dma.c , and lab4.c code are intended to make your interaction with the hardware much simpler. These files handle the the buffering of data using interrupts from the CODEC (A/D and D/A) and the USB port. Here, wewill describe the important aspects of the code necessary to complete the assignment.

In the first few labs, data was processed on a sample-by-sample basis, so no buffering was necesary. However, the spectrum analyzer to be implemented in this lab works over a block of $N=1024$ samples. If it were possible to compute a 1024-point FFT in the sample time of onesample, then no additional interrupt handling routines would be necessary. Samples could be collected ina 1024-length buffer and a 1024-point FFT could be computed uninterrupted while the buffer fills.Unfortunately, the DSP is not fast enough to accomplish this task.