8
Unfolding and Folding of Architectures
8.1 Introduction
Major decisions in digital design are based on the ratio of sampling clock to circuit clock. The sampling clock is specific to an application and is derived from the Nyquist sampling criteria or band-pass sampling constraint. The circuit clock,on the other hand, primarily depends on the design and the technology used for implementation. In many high-end applications, the main focus of design is to run the circuit at the highest possible clock rate to get the desired throughput. If a simple mapping of the dataflow graph (DFG) on hardware cannot be synthesized at the required clock rate, the designer opts to use several techniques.
In feed forward designs, an unfolding transformation makes parallel processing possible. This results in an increase in throughput. Pipelining is another option in feed forward design for better timing. Pipelining is usually the option of choice as it results in a smaller area than with an unfolded design. In feedback Digs, the unfolding transformation does not result in true parallel processing as the circuit clock rate needs to be reduced and hence does not give any potential iteration period bound (IPB) improvement. The only benefit of the unfolding transformation is that the circuit can be run at slower clock as each register is slower by the unfolding factor. In FPGA-based design, with a fixed number of registers and embedded computational units, unfolding helps in optimizing designs that ...
