Chapter 15
Case Study
This case study brings together all the principles established in the rest of the book. It covers the RTL design of a synchronous system, the design of test benches to test it and the results from those tests.
The example is a low-pass digital filter and the case study shows how the original specification is gradually converted into an RTL design suitable for logic synthesis, with an exploration of the design space to find an optimum data representation and precision for the filter calculations.
The example also shows how test benches are written for this design that explore the frequency response of the filter and verify that the specification has been met.
15.1 Specification
The specification is a Low-Pass digital filter with the following characteristics:
Maximum Frequency: 80 kHz;
Cut-off Frequency: 20 kHz;
Transition Bandwidth: 10 kHz;
Stop-band Attenuation: 18 dB.
The maximum frequency is the highest input frequency that the filter can handle.
The cut-off frequency is the end of the pass band and the frequency at which the gain should start to drop off.
The transition bandwidth is the frequency gap between the cut-off frequency at the end of the pass band and the beginning of the stop band.
The stop-band attenuation is the reduction in signal power for frequencies in the stop-band.
For this example the cut-off frequency is set at 20 kHz, the transition bandwidth is 10 kHz, so the stop band starts at 30 kHz at which point the specification requires ...
