FPGA Introduction

Hao wang and Jyh-Charn (Steve) Liu

What is an FPGA?

• Field Programmable Gate Array– Configurable Logic blocks (CLB), interconnection

resources, and I/O pads.

How FPGA works?

• CLB consists of two slices, each of which contains look-up tables (LUT), registers, multiplexers, and carry logic.– LUT implement logic functions– Registers store data– Multiplexers select the desired output– Carry logic enables fast arithmetic function

• Interconnections are routing resources including channels, switch boxes, clock distribution networks, etc.

CLB architecture

• Arrangement of slices within the CLB

Slice diagram

Slice diagram

How LUT works• An 2-input LUT configured as follows implements AND gate

• It can be easily reconfigured to implement OR, XOR, NAND, and NOR gates, which are the basics to build up more complex functions.

• Modern FPGAs have 6-input LUTs which can be cascaded to implement complex digital circuits.

Input 1 Input 2 Output

0 0 0

0 1 0

1 0 0

1 1 1

Interconnection architecture

• Switch box topology

Interconnection architecture

• Interconnection patterns

Which takes more chip area, logic or interconnects?

Logic: 20% ~ 30%Interconnect: 70% ~ 80%

How to Reconfigure FPGA

• Logic block functions and interconnections are specified using Hardware Description Language (HDL).

• The HDL code is synthesized, mapped, placed ,routed and download onto the chip by vendor-provided tools

Gate-levelnetlist

BEGIN CIRCUIT=TEST INPUT SET_A, SET-B, DATA, CLOCK, CLEAR_A, CLEAR_B; OUTPUT Q, N_Q; WIRE SET, N_DATA, CLEAR;

GATE G1=NAND (IN1=SET_A, IN2=SET_B, OUT1=SET); GATE G2=NOT (IN1=DATA, OUT1=N_DATA); GATE G3=OR (IN1=CLEAR_A, IN2=CLEAR_B, OUT1=CLEAR); GATE G4=DFF (IN1=SET, IN2=N_DATA, IN3=CLOCK, IN4=CLEAR, OUT1=Q, OUT2=N_Q);

END CIRCUIT=TEST;

Fully-routed physical(CLB-level) netlist

Schematiccapture

Mapping

Packing

Place-and-Route Timing analysis

and timing report

Gate-level netlistfor simulation

SDF (timing info)for simulation

How to Reconfigure FPGA

• Each time a new configuration is downloaded, the FPGA behaves like a new chip implementing a new function.

• The configuration is lost when powered off, because theyare stored in SRAM.– However, the configuration file can be stored in external storage such

as flash card or PROM so that the chip can automatically load it when powered on

Why use FPGA?

• Compared to CPU, FPGA has the following benefits– Performance– Throughput– Reliability

• Compared to ASIC, FPGA has the following benefits– Reconfigurability– Cost– Time to market

Benefit over CPU: Performance

• FPGA code runs in real time in nanoseconds.• All the logic blocks inside FPGA can run

concurrently in parallel, since they are real hardware circuits.

• If fully utilized, FPGA can implement a high performance many-core system.

Benefit over CPU : Throughput

• FPGA has many high speed I/O pins to interface with memory, Ethernet, etc. It has dedicated channels for peripherals, unlike CPU whose peripherals share a bus.

• FPGA vendors provide many high speed communication IP cores.

Benefit over CPU : Reliability

• Software programs are usually built upon several layers of abstractions (driver, OS, etc) to help schedule tasks and share resources. They are at risk of incompatibility, resource contention, deadline violation, etc.

• FPGA circuitry is a hard implementation, which is deterministic and well predictable.

Benefit over ASIC : Reconfigurability

• ASIC can only do a specific job. If you want an extra function after an ASIC has been manufactured, design another one! While for FPGA, that is nothing but adding a piece of code and re-compile it.

• Certain bugs are caught after the ASIC has been manufactured and distributed to customers. A recall may put the company into bankrupt. While for FPGA, developers can fix the bug in the HDL code and distribute the update to customers.

Benefit over ASIC : Cost

• ASIC development is usually non-recurring engineering. Product fabrication also costs a lot of money.

• FPGA vendors produce large volume of chips so that end users do not need to eat the fabrication cost. Reconfigurability of FPGA makes it reusable for different projects.

Benefit over ASIC : Time to market

• ASIC needs cost many man-years to design, test, validate, and fabricate.

• FPGA development boards usually come with a rich set of peripherals and IP cores which makes it ideal for rapid prototyping.

• Questions?

Reference

• Introduction to FPGA Technology: Top 5 Benefits. http://www.ni.com/white-paper/6984/en#toc1

• FPGA introduction, Ovind Harboe• http://www.fpga4fun.com/FPGAinfo1.html• http://en.wikipedia.org/wiki/Field-programmable_gate_

array#Architecture• The Design Warrior's Guide to FPGAs• Virtex-6 FPGA Configurable Logic Block User Guide• http://www.cse.unsw.edu.au/~cs4211/seminars/va/Virt

exArchitecture.html• http://www.1-core.com/library/digital/fpga-logic-cells/• http://www.ni.com/white-paper/7440/en