Motorola MSC8101 ADS User's Guide download pdf (Page 299)

Glossary Appendix B-9

FIR

Finite impulse response. A type of filter. FIR filters are characterized

by transfer functions that are polynomials, where the coefficients are

directly the impulse response of the filter. The most common

implementation of an FIR filter is the direct form with the recursive

section removed. The form of an FIR filter gives rise to the

terminology of tapped delay line and the coefficients as tap weights.

The length of an FIR filter is the number of taps, N, and thus the

convention of using indices from 0 through (N–1) for the coefficients.

FIR filters have two main advantages: they are inherently stable and

the finite length of the impulse response guarantees that the output

will go to zero within N samples from the epoch the input goes to

zero. Another advantage is that filters with a precisely linear-phase

characteristic can be designed and implemented. The disadvantage of

FIR filters is their computational complexity; for each output sample,

N multiply-accumulate (MAC) operations must be performed. For the

most part a target (magnitude) frequency response can be

approximated using IIR filters of less computational complexity,

expressed in MACs per input sample, than FIR filters. However, if

there is a sampling rate change, then FIR filters can be constructed to

compute only those output samples that are really necessary, thus

reducing the computational burden. If we allow the FIR filter to

operate on data in blocks, rather than sequentially on a

sample-by-sample basis, there are other techniques to reduce the

number of MACS per output sample.

flyby transfer

Also known as a “single access transaction.” The data path is between

a peripheral and memory with the same port size, located on the same

bus. On the MSC8101, flyby transactions can occur only between

external peripherals and external memories located on the PowerPC

bus, or between internal peripherals and internal SRAM located on the

local bus. Flyby operations do not require access to the DMA FIFO.

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Motorola MSC8101 ADS User's Guide Page 299