FPGA Switch Block Design Dr. Philip Brisk Department of Computer Science and Engineering University of California, Riverside CS 223.

FPGA Switch Block Design

Dr. Philip BriskDepartment of Computer Science and Engineering

University of California, Riverside

CS 223

FPGA Architecture (Recap)

Routing Instance and an S Block

Flexibility of Interconnection Structures for Field-

Programmable Gate Arrays

J. Rose and S. Brown,IEEE Journal of Solid-State Circuits 25(3): 277-282,

Mar. 1991

Key Questions

• What is the effect of C Block flexibility on routing completion rate?

• What is the effect of S Block flexibility on routing completion rate?

• How do S and C Block flexibilities interact?

• What is the effect of S and C Block flexibilities on the number of tracks per channel to achieve 100% routability?

Switch Block Flexibility• Total number of possible connections offered

to each incoming wire

Switch Block Routability

• Cannot route from A to B

• Can route from A to B– Assymmetric about

horizontal and vertical axes

Fs = 2Fs = 2

Example Connection Block

Routability Study (One Benchmark)

W = 14

• Increasing FS improves routability, but FC must be high to achieve 100% routability

• Routing completion rate approaches 100% when FC > ½W

• Routing completion rate is low for low values of FC

Routability Study (One Benchmark)

W = 14

• If FC is high enough, then low values of FS can achieve 100% routability

• The number of different paths between the initial physical pin and the terminating C Block of a two-pin wire is given by:

where N is the number of S Blocks on the global path

• For lower values of FC, increasing FS improves routability up to a point

S Block vs. C Block FlexibilityAv

g. F

C/W

for 1

00%

routi

ng c

ompl

etion • A more flexible

S Block can compensate for a less flexible C Block

Track Count Requirement for BNREto Achieve 100% Routability

Conclusion

• C Blocks should have high flexibility to achieve high-percentage routing completion

• S Blocks require limited flexibility

• With low flexibility, only a few extra tracks more than the minimum can achieve 100% routability

Universal Switch Modules for FPGA Design

Y-W. Chang et al.,ACM Transactions on Design Automation for Electronic Systems 1(1): 80-101, Jan. 1996

Overview

• A Switch Block with larger routing capacity has better area-performance in FPGA routing– Increased connectivity of routing components– Equivalence of LUT/CLB inputs permits pin

permutations, which yields highly optimal routing– Most nets are short

• 60% of nets route through at most 2 Switch Blocks• 90% of nets route through at most 5 Switch Blocks

• Tradeoff between routing capacity and area

Universal Switch Module DefinitionRouting Resource Vector (RRV):N = (n1, n2, n3, n4, n5, n6), 0 < ni < W

Example: N = (1, 0, 1, 1, 0, 0) is routable on the following:

n1

n4 n3

• A Switch Block of size W is universal if the following inequalities are sufficient to determine of an RRV is routable:

Examples

Universal Sub-modules

• A sub-module of a Universal switch is also universal (but for a smaller W)

Theoretical Results

• A universal S Block can be constructed with at least 6W switches

• Any S Block constructed with less than 6W switches cannot be universal

Non-universal S Blocks

Disjoint Switch Block (Xilinx XC4000 series)

Antisymmetric Switch Block (Rose and Brown, 1991)

Channel Width Required for 100% Routing Capacity (One Benchmark)

Conclusion

• Universal S Blocks offer better routability than disjoint and antisymmetric S Blocks

• Algorithm presented to generate S Blocks that are universal (not discussed)

Architectures and Algorithms for Field-Programmable Gate Arrays

with Embedded Memory

S. Wilton,Ph.D. Thesis, University of Toronto, 1997

(Section 6.1.2)

S Blocks

Disjoint Universal Wilton

Start with Universal S Block, and rotate the diagonal connections by one track

FPGA Routing Structures: A Novel Switch Block and Depopulated

Interconnect Matrix Architectures

M. I. Masud,M.S. Thesis, University of British Columbia, 1998

Routing with a Disjoint S Block

• Routing fabric partitioned into domains• Cannot cross domains (using routing only)

Routing with a Wilton S Block

• Eliminates domain choice problem• Many more routing choices are available

Implementation Details

WiltonDisjoint

WiltonDisjoint

Area Overhead

Imran S Block

• Routability of Wilton S Block• Implementation efficiency of Disjoint S Block

Imran S Block(1) Tracks that terminate at the S Block

• Wilton topology

(2) Tracks that pass through the S Block• Disjoint topology

Area Results

Delay Results

Channel Width Results

Conclusion

• Imran Switch Block– Routability of Wilton Switch Block– Area-efficiency of Disjoint Switch Block