Verification of Parameterised FPGA Circuit Descriptions with Layout ...

CHAPTER 2. BACKGROUND AND RELATED WORK 9
Structural hardware descriptions in VHDL or Verilog can be annotated with “RLOC” place-
ment constraints for targeting Xilinx FPGA architectures. Figure 2.1 illustrates how this
can be used in VHDL to place a column of flip-flops. The placement co-ordinates are given
using the co-ordinate system for Virtex and earlier Spartan devices, where positions are
given as a string of “RmCn.Sp” to place a component in a particular row and column and
then at a numbered slice at that position. Later device families, including Virtex-II, use a
slice-based co-ordinate system where components are placed with a co-ordinate scheme of
“XmYn” where each combination of (m, n) describes a different slice.
The Pebble language [46] supports a simpler but equivalent placement infrastructure by
allowing hardware instantiations to be annotated with at (x,y). These co-ordinates are
mapped into an appropriate architecture-specific format when the Pebble descriptions are
compiled into VHDL [48].
Specifying layouts with absolute co-ordinates is tedious and error-prone, particularly for
parameterised hardware descriptions where placement constraints may work for some com-
binations of parameters but not others. For parameterised, placed, hardware libraries this is
a particular issue since even if the design if functionally correct some instantiations of it may
not produce valid layouts.
An alternative is relative placement, where components are instantiated below or beside one
another. Relative placement is not as powerful as placement with explicit co-ordinates, how-
ever if properly specified then it does remove the possibility of describing incorrect layouts.
The Pebble system has been extended to support relative placement [49, 50]. All block
instantiations must be contained within a beside or below block which describes layout on a
grid. beside for and below for constructs are provided to handle iteration. The Pebble system
is unique in that relatively placed circuit descriptions can be compiled into parameterised
libraries with explicit co-ordinates, however it does not always do this in the most optimal
manner as we will discuss in Chapter 3.
Relative placement systems based on higher-order combinators have been demonstrated for
Lava [7] and Ruby [24, 26, 31]. Ruby circuits are described as relations between inputs
and outputs and can be given layout interpretations through their use of beside and below