1. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Topics n Circuit design for FPGAs: –Logic elements. –Interconnect.

2. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Multiplexers as logic elements 1 1 0 A A 1 B 0 (AB)’1 0 0 A A 1 B 0 A^BQ 0 D CLR 0 0 CLK latch

3. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Using antifuses

4. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Static CMOS gate vs. LUT n Number of transistors: –NAND/NOR gate has 2n transistors. –4-input LUT has 128 transistors in SRAM, 96 in multiplexer. n Delay: –4-input NAND gate has 9  delay. –SRAM decoding has 21  delay. n Power: –Static gate’s power depends on activity. –SRAM always burns power.

5. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Lookup table circuitry n Demultiplexer or multiplexer? LUT adrs LUT adrs

6. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Traditional RAM/ROM n Cell drives long bit line: Bit line adrs

7. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Lookup memory n Multiplexer presents smaller load to memory cells. –Allows smaller memory cells.

8. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Multiplexer styles static gates pass transistors

9. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Multiplexer design n Pass transistor multiplexer uses fewer transistors than fully complementary gates. n Pass transistor is somewhat faster than complementary switch: –Equal-strength p-type is 2.5X n-type width. –Total resistance is 0.5X, total capacitance is 3.5X. –RC delay is 0.5 x 3.5 = 1.75 times n-type switch.

10. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Static gate four-input mux n Delay through n- input NAND is (n+2)/3. n Lg b + 1 inputs at first level, so delay is (lg b + 3)/3. n Delay at second level is (b+2)/3. n Delay grows as b lg b.

11. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Pass-transistor-based four-input mux n Must include decode logic in total delay.

12. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Tree-based four-input mux n Delay proportional to square of path length. n Delay grows as lg b 2.

13. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR LE output drivers n Must drive load: –Wire; –Destination LE. n Different types of wiring present different loads.

14. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Avoiding programming hazards n Want to disable connections to routing channel before programming. From LE Routing channel config progb

15. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Interconnect circuits n Why so many types of interconnect? –Provide a choice of delay alternatives. n Sources of delay: –Wires. –Programming points.

16. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Styles of programmable interconnection point pass transistor Three-state

17. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Pass transistor programmable interconnect point n Small area. n Resistive switch. n Delay grows as the square of the number of switches.

18. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Three-state programmable interconnection point n Larger area. n Regenerative driver. +

19. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Switch area * wire delay vs. buffer size (Betz & Rose) © 1999 IEEE

20. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Switch area * wire delay vs. pass transistor width (Betz & Rose) © 1999 IEEE

21. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Wire delay vs. switch sizes (Chandra and Schmidt) n Delay vs. switch size for various driver sizes. n U-shaped curve: –Resistance initially decreases. –Increased capacitance eventually dominates. © 2002 IEEE

22. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Clock drivers n Clock driver tree:

23. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR Clock nets n Must drive all LEs. n Design parameters: –number of fanouts; –load per fanout; –wiring tree capacitance. n Determine optimal buffer sizes.

24. FPGA-Based System Design: Chapter 3 Copyright  2004 Prentice Hall PTR H tree n Regular layout structure. –Recursive.