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Verification of Parameterised FPGA Circuit Descriptions with Layout ...

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CHAPTER 6. LAYOUT CASE STUDIES 145<br />

gr_lut<br />

gr_lut<br />

gr_lut<br />

gr_lut<br />

gr_lut<br />

gr_lut<br />

eq_lut<br />

eq_lut<br />

eq_lut<br />

eq_lut<br />

eq_lut<br />

eq_lut<br />

mux mux<br />

mux mux<br />

mux mux<br />

mux mux<br />

mux mux<br />

mux mux<br />

Figure 6.12: 6-bit 2-sorter circuit<br />

be placed anywhere <strong>with</strong>in the rcomp parameter composition since the type is polymorphic,<br />

however the logical place to put the spacer in order to put room between each butterfly stage<br />

is next to the map instantiation - it can then be given the desired width and any height (less<br />

than the expected height <strong>of</strong> the map instantiation) and the series composition layout will<br />

ensure that this space is left free.<br />

6.4.2 Implementing a bitonic merger<br />

The butterfly circuit we evaluate is a network <strong>of</strong> 2-sorters. This is a bitonic merger circuit<br />

which merges together two sorted lists. The merger is bitonic because the order <strong>of</strong> the input<br />

lists must be opposed – i.e. if one is ascending then the other must be descending or vice<br />

versa.<br />

We design a 2-sorter circuit which operates on n-bit data values and lay it out as a 4 × n<br />

block as shown in Figure 6.12. The first two columns are a comparator which outputs a<br />

control signal to the multiplexers to select the maximum and minimum values.<br />

The butterfly sorting network can be pipelined by inserting registers between each stage,<br />

replacing the R block by R ; D We can state the correctness <strong>of</strong> a pipelining arrangement<br />

<strong>with</strong> the following theorem:<br />

Theorem 24 R ; D = D ; R ⇒ butterfly n R = butterfly n (R ; D) ; D −n<br />

Pro<strong>of</strong> This requires a lemma about repeated series composition:

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