FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Moore’s Law n Gordon Moore: co-founder of Intel. n Predicted that number of transistors per chip would grow exponentially (double every 18 months). n Exponential improvement in technology is a natural trend: steam engines, dynamos, automobiles.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Moore’s Law plot

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR The cost of fabrication n Current cost: $2-3 billion. n Typical fab line occupies about 1 city block, employs a few hundred people. n New fabrication processes require 6-8 month turnaround. n Most profitable period is first 18 months-2 years.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Cost factors in ICs n For large-volume ICs: –packaging is largest cost; –testing is second-largest cost. n For low-volume ICs, design costs may swamp all manufacturing costs. –$10 million-$20 million.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Mask cost vs. line width

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Field-programmable gate arrays n FPGAs are programmable logic devices: –Logic elements + interconnect. –Provide multi-level logic. LE Interconnect network LE

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR FPGAs and VLSI n FPGAs are standard parts: –Pre-manufactured. –Don’t worry (much) about physical design. n Custom silicon: –Tailored to your application. –Generally lower power consumption.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Standard parts vs. custom n Do you build your system with an FPGA or with custom silicon? –FPGAs have shorter design cycle. –FPGAs have no manufacturing delay. –FPGAs reduce inventory. –FPGAs are slower, larger, more power-hungry.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Challenges in system design n Multiple levels of abstraction: logic to CPUs. n Multiple and conflicting constraints: low cost and high performance are often at odds. n Short design time: Late products are often irrelevant.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR The system design process n May be part of larger product design. n Major levels of abstraction: –specification; –architecture; –logic design; –circuit design; –layout. FPGA-based system design

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Elements of an FPGA fabric n Logic. n Interconnect. n I/O pins. … LE interconnect IOB …

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Terminology n Configuration: bits that determine logic function + interconnect. n CLB: combinational logic block = logic element (LE). n LUT: Lookup table = SRAM used for truth table. n I/O block (IOB): I/O pin + associated logic and electronics.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Logic element n Programmable: –Input connections. –Internal function. n Coarser-grained than logic gates. –Typically 4 inputs. n Generally includes register. n May provide specialized logic. –Adder carry chain.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Example logic element n Lookup table: about memory a b out

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Logic synthesis n How do we break the function into logic elements? n How do we implement an operation within a logic element?

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Placement n Where do we put each piece of logic in the array of logic elements? … LE

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Programmable wiring n Organized into channels. –Many wires per channel. n Connections between wires made at programmable interconnection points. n Must choose: –Channels from source to destination. –Wires within the channels.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Programmable interconnection point DQ

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Programmable wiring paths

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Choosing a path LE

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Routing problems n Global routing: –Which combination of channels? n Local routing: –Which wire in each channel? n Routing metrics: –Net length. –Delay.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Segmented wiring Length 1 Length 2

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Offset segments

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR I/O n Fundamental selection: input, output, three- state? n Additional features: –Register. –Voltage levels. –Slew rate.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Programming technologies n SRAM. –Can be programmed many times. –Must be programmed at power-up. n Antifuse. –Programmed once. n Flash. –Similar to SRAM but using flash memory.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Configuration n Must set control bits for: –LE. –Interconnect. –I/O blocks. n Usually configured off-line. –Separate burn-in step (antifuse). –At power-up (SRAM).

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Configuration vs. programming n FPGA configuration: –Bits stay at the device they program. –A configuration bit controls a switch or a logic bit. n CPU programming: –Instructions are fetched from a memory. –Instructions select complex operations. CPUmemory add r1, r2IRadd r1, r2

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Reconfiguration n Some FPGAs are designed for fast configuration. –A few clock cycles, not thousands of clock cycles. n Allows hardware to be changed on-the-fly.

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR FPGA fabric architecture questions n Given limited area budget: –How many logic elements? –How much interconnect? –How many I/O blocks?

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Logic element questions n How many inputs? n How many functions? –All functions of n inputs or eliminate some combinations? –What inputs go to what pieces of the function? n Any specialized logic? –Adder, etc. n What register features?

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR Interconnect questions n How many wires in each channel? n Uniform distribution of wiring? n How should wires be segmented? n How rich is interconnect between channels? n How long is the average wire? n How much buffering do we add to wires?

FPGA-Based System Design: Chapter 1 Copyright  2004 Prentice Hall PTR I/O block questions n How many pins? –Maximum number of pins determined by package type. n Are pins programmed individually or in groups? n Can all pins perform all functions? n How many logic families do we support?