High-Level Interconnect Architectures for FPGAs

1. High-Level Interconnect Architectures for FPGAs Nick Barrow-Williams

2. Introduction • Semiconductor industry has grown rapidly for several decades • Continued shrinking of device dimension introduces new design challenges • Moving data around a chip can now be the limiting factor of performance • Existing solutions do not scale well

3. Why do existing solutions not scale? • Global connections are longer • Wire depth increased to counter width decrease • Parasitic capacitive effects increase and cause slow signal propagation

4. Why do existing solutions not scale? • Existing system-level connection uses buses • Buses increase resource efficiency and decrease wiring congestion • Not suitable for a large number of modules • A network based alternative would offer higher aggregate bandwidth

5. Why design for FPGA systems? • FPGA silicon area already dominated by wiring • Global wires are limited in number • Increasing gate count only increases wiring congestion

6. The Solution: Network-on-Chip • Use technologies from network systems • Replace inefficient global wiring with high-level interconnection network • Create scalable systems to handle large numbers of modules

7. Existing Solutions • Most existing systems are for ASIC designs • Stanford Interconnect • RAW • SCALE • SPIN • PNoC: An solution for FPGAs • Complex • High hardware cost • Other simulated solutions exist but few are implemented

8. Proposal: Two network systems • Existing solutions use either packet switching or circuit switching techniques • Design, implement, test and synthesise one of each to compare performance and hardware cost • Map solutions to an FPGA platform to evaluate hardware cost in current generation systems

9. Network Architecture Design • Topology • Simple • Scalable • 2 Dimensional • Solution: 2D mesh Topology

10. Network Architecture Design • Routing Algorithm • Deterministic • Data always follows same path through network • Simple hardware • Sensitive to congestion • Adaptive • Paths through network can change according to load • Complex hardware • Avoids congestion

11. Network Architecture Design • When choosing routing algorithms must avoid: • Deadlock: • Livelock Solution: Use unidirectional wiring and allow each node to make two connections Solution: Use deterministic routing

12. Network Architecture Design • Flow control methods • Circuit switched • Circuit request propagates through network • Path reserved to destination • Grant signal propagates back • Data sent then circuit deallocated • Packet switched • Use header, body and tail • Wormhole routing • Forward header and body without waiting for tail • Need buffers to store stalled packets

13. Router Design • Each router contains a number of modules • FIFOs (only present in packet switched router) • Address to port-request decoder • Arbiter • Control finite state machines • Crossbar

14. Circuit Switched Router Structure Data In Arbiter Address to Port Decoder Request In In & Out Ports FSM Request Out Request In Grant Out Grant In Data In Crossbar Data Out

15. Packet Switched Router Structure Data From FIFOs Arbiter Address to Port Decoder Request From FIFOs Write FIFO FSM Req Data In Grant Full Data In & Out Ports Control Write Out Request In 5 Queue Modules Grant Out Full In Data From FIFOs Crossbar Data Out

16. Router Implementation and Testing • Both routers were coded using VHDL • Simulation and testing used a combination of ModelSim and Xilinx ISE 9.1 • Ad-hoc tests used for individual modules • VHDL testbench used for system verification

17. Testbench Structure TESTBENCH Clock Gen Reset Gen Cycle Count Mesh Network Input Tables #START SOURCE DEST SIZE ID # ------------------------------------------------------ 2 3 0 0 1 8 1 3 2 0 0 1 2 2 3 2 3 1 1 2 3 4 3 1 1 0 8 4 5 0 3 1 3 7 5 Source Sink Command File Output Table Test Table Read Input Compare Output File Success: ID: 1 Source : (0,3) Dest : (1,0) Hops : 4 Latency: 34 Success: ID: 2 Source : (0,2) Dest : (1,0) Hops : 3 Latency: 27 Success: ID: 3 Source : (3,2) Dest : (1,1) Hops : 3 Latency: 22 Success: ID: 4 Source : (1,3) Dest : (0,1) Hops : 3 Latency: 22 Success: ID: 5 Source : (3,0) Dest : (3,1) Hops : 1 Latency: 12

18. Synthesis • Each router was synthesised for a Virtex-4 LX platform • Post-synthesis verification • Resource usage • Timing

19. Circuit Switched Resource Usage LUTs Flip-Flops Total of 586 4 Input LUTS ~0.1% of a Virtex 5 Total of 202 Flip Flops

20. Packet Switched Resource Usage LUTs Flip-Flops Total of 786 4 Input LUTS +34% compared to circuit switched Total of 237Flip Flops

21. Timing Results Circuit Switched Packet Switched • Max Freq 126.330MHz • Setup time 5.308ns • Hold time 0.272ns • Max Freq 144.533MHz • Setup time 6.125ns • Hold time 0.272ns Critical path is through Arbiter in both designs

22. Project Appraisal • Maintaining an accurate software simulation proved difficult • A great deal was learnt during the implementation of the circuit switched network • HDL implementations are only prototypes • Testbench provides a good framework but more time is needed to gather performance data

23. Conclusions • Possible to make low complexity network-on-chip systems suitable for FPGAs • Latency has to be traded for throughput • Hard to collect performance data without application driven benchmarks • Both networks are viable so why not use both?

24. Future Work • Cycle accurate software simulations • Application driven benchmarking • Serial transmission • Power efficiency • Industry standard solution