3.6 Assembly implementation of fir filters on ti tms320c62x

Implementation of fir filters in assembly

Storing filter coefficients

Rather than defining the filter coefficients in your main assembly program file, it is usually more convenient tostore them in a separate file. By defining the coefficients in a separate assembly (for example, coeff.asm ) file, you can load the coefficients at a desired memory location at the run time,although it is not essential for the current simple FIR filtering lab.

The assembly file containing the filter coefficients can be written as follows:

Each coefficient must be converted to the Q-15 format and defined by each .short assembly directive. For your convenience, I wrote a short MATLABscript save_coef.m that converts the filter coefficients stored as a MATLAB vector to Q-15 format andthen writes to a file exactly in the above format. (You can download save_coef.m from the course web page.) The section coeffs should be defined in the link command file so that the coefficients are tobe loaded at the correct memory location.

You can simply include the coeff.asm using the .include directive at the beginning of your main assembly program.

Assembly implementation

Based on the codec input and output program you have written in the previous labs, you can now implement areal-time FIR filtering algorithm.

Implementation using circular addressing modes

As you might already have noticed, a lot of cycles are wasted in FIR filtering while maintaining the buffer tosee if you reached the end of buffer and update the address pointers properly. To avoid this unnecessarybuffer maintenance, the TI DSP processors have a special addressing mode, called circular addressing . Using circular addressing, you can define a block ofmemory as a circular buffer. As you increase (or decrease) the pointer register pointing to the buffer index beyondthe buffer limit, it automatically points to the other end of the buffer, implementing a circle of data array.Instead of moving the data samples themselves, you can move the pointer which specifies the beginning of thebuffer, as each new sample is processed. You don't need to check if you reached the end of buffer because the addresspointer returns to the beginning of the buffer immediately after reaching the end.

Of the 32 registers on the C6x, 8 of them can perform circular addressing. These registers are A4 through A7and B4 through B7. Since circular addressing is not default, each of these registers must be specified ascircular using the AMR (Address Mode Register) register. The lower 16 bits of the AMR are used to select the mode for each of the 8 registers. The upper 10 bits (6 are reserved) areused to set the length of the circular buffer. Buffer size is determined by $2^{(N+1)}$ bytes, where $N$ is the value appearing in the block size fields of the AMR register. The top address of the buffer needs to be aligned with proper physical memory blockaddress using the .align assembler directive.