L2: FPGA HARDWARE 18-545: ADVANCED DIGITAL DESIGN PROJECT FALL 2016 BRANDON LUCIA
Admin stuff Project Proposals happen on Monday Be prepared to give an in-class presentation Lab 1 is due Wednesday, Sept. 14th Reading Assignment #1 due today Submit a PDF/text file, don't fill in the web form Team assignments are done 18-545: FALL 2016 2
Admin Stuff Status reports due today No word docs, please! Be specific about what happened/is going to happen Talk about what YOU did/will do, not just what your group did Grades on the way, as general feedback 18-545: FALL 2016 3
Game Plan Overview Why use FPGAs? FPGA Internals Caveat: I will use Xilinx specific terminology since that’s the FPGA company you will be using. Beware that other companies use different terms 18-545: FALL 2016 7
FPGA Overview F ield P rogrammable G ate A rray Array of generic logic gates Gates where logic function can be programmed Programmable interconnection between gates Fielded systems can be programmed i.e. post-fabrication
Xilinx Virtex-5 FPGA 18-545: FALL 2016 9
Design Platform Virtex-5 Development System Xilinx XC5VLX110T FPGA 17280 slices of CLB goodness 256MB DDR2 (SODIMM) DVI Video port VGA port is for input 10/100/1000 Ethernet port Audio Codec (AC97) USB2 port 16x2 LCD, RS-232 Compact Flash card slot Expansion connectors 18-545: FALL 2016 10
Game Plan Overview Why use FPGAs? FPGA Internals 18-545: FALL 2016 11
Why use FPGAs? System designers have a Goldilocks problem Off-the-shelf parts are not efficient enough Custom ASICs cost too much Need a “just right” solution
ASIC Design Difficult to design Large and complex Issues in advanced processes Interconnect delay Device leakage Power density constraints Expensive to design / fabricate Mask set costs Non-recurring engineering costs Need a high-volume, high-profit market to justify costs!
Energy Efficiency (MOPS/mW) Area Efficiency (MOPS/mm2) 10000 Microprocessors 1000 100 10 1 0.1 DSPs ASICs 0.01 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Efficiency View An efficiency gap exists between ASICs and CPUs N. Zhang, et. al, “The Cost of Flexibility in Systems on a Chip Design for Signal Processing Applications”
Decreasing FPGA unit Development Cost + Device Cost cost pushing crossover point to the right ASIC solution has a lower total cost ASIC Trend Additional ASIC costs: • Increasing NRE charge • 58% are late to market -- impacts total volumes shipped FPGA solution has a lower total cost • ASIC cycle longer than some market windows • Over 50% need to be respun FPGA Trend Total Units (Courtesy Xilinx, Inc.) Economic View FPGAs: High package costs ($300+), low NRE costs ASICs: Low package costs (pennies), high NRE costs ($600K+)
FPGA Advantages Higher performance than CPU solution Lower power than CPU solution (usually) Low NRE costs Off-the-shelf part designed by FPGA vendor You are sharing NRE costs with all other customers Fast design time Low time-to-market Fast re-design / re-fabrication time Easy to correct an error, to add functionality, in response to spec change Can even change product after deployment 18-545: FALL 2016 16
FPGA Disadvantages High per-part costs Good for low to middle volume applications High volume applications should consider ASICs Perhaps use FPGA for prototyping Lower performance than ASIC Higher power than ASIC More specialized design skills than programming a CPU 18-545: FALL 2016 17
Example uses of FPGAs Rapid Prototyping Emulation of ASIC design Design exploration Shipping product Networking Military Microsoft Bing Datacenters Reconfigurable Computing Research! (http://parallel.princeton.edu/openpiton/)
Game Plan Overview Why use FPGAs? FPGA Internals 18-545: FALL 2016 19
FPGA Breakdown 3 Basic components Configurable Logic Blocks General purpose interconnect I/O Blocks Advanced components Hard macros CPUs Block RAM Multipliers Specialized components DSP blocks VIRTEX-II PRO
XILINX XC3020 I/O BLOCK (64 TOTAL ) CLB (64 TOTAL) GENERAL IOBS HAVE DIRECT PURPOSE ACCESS TO INTERCONNECT ADJACENT CLBS SWITCH MATRIX (COURTESY XILINX, INC.)
ROUTING EVEN MORE ZOOMED IN VIEW ZOOMED IN VIEW OF THE CLB MATRIX OF THE FPGA SPECIFIC INGRESS AND EGRESS CONNECTION OPTIONS (BLACK DOTS) ARE AVAILABLE (COURTESY XILINX, INC.)
ROUTING: THE SWITCH MATRIX EACH MATRIX HAS 5 CONNECTIONS PER SIDE (COURTESY XILINX, INC.)
ROUTING: THE SWITCH MATRIX EACH MATRIX HAS 5 CONNECTIONS PER SIDE ONLY CERTAIN CONNECTION PATTERNS ARE POSSIBLE (COURTESY XILINX, INC.)
Hierarchical Routing Spartan-2 and more recent have different length connections between switch matrices Local roads, limited access roads, interstate highways Routes across entire chip don’t burn lots of short connections 18-545: FALL 2016 25
Detailed Routing (Spartan 2)
Configurable Logic Blocks CLBs get more and more stuff crammed in them over time XC3K family had LUT (5 variable input, 2 FF values, 2 outputs), 2 FFs, clock enable, FF reset (direct / global) and 9 muxes ~51 bits of configuration SRAM per CLB (COURTESY XILINX, INC.)
What’s a Look -up-table (LUT)? A direct implementation of a truth table, using memory LUT inputs are memory address values LUT outputs are the memory data value A B C D F A B C D F 0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 0 0 0 1 1 0 0 1 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 1 1 0 1 0 0 0 0 1 0 0 1 0 1 0 1 1 0 1 0 1 1 0 1 1 0 0 0 1 1 0 1 0 1 1 1 1 0 1 1 1 1 1 0 0 0 0 1 0 0 0 1 1 0 0 1 1 1 0 0 1 1 1 0 1 0 0 1 0 1 0 1 1 0 1 1 0 1 0 1 1 1 1 1 0 0 1 1 1 0 0 0 1 1 0 1 1 1 1 0 1 0 1 1 1 0 0 1 1 1 0 0 1 1 1 1 0 1 1 1 1 0 18-545: FALL 2016 28
Another View of LUTs Can view LUT as 16:1 mux Inputs are mux select Config sets mux data inputs Logically same as 16x1 memory Can compact logic if you can route inputs to mux data inputs 18-545: FALL 2016 29
Look Up Table Additional Functionality Can be configured as: Shift register (16 regs) Small memory (16 bits) “Distributed RAM” Some other FPGAs use muxes instead of memories to implement the core combinational logic
Spartan-2 CLB Spartan-2 has 2 LUTs (4 input each) feeding a 3rd LUT, 2 FFs (with Preset/Reset, Enable, posedge or negedge clocks) and 16 muxes 12 inputs (plus clock), 4 outputs (COURTESY XILINX, INC.) 18-545: FALL 2016 34
Spartan-3 CLBs are composed of 4 slices Organized as 2 pairs, one of which is optimized for memory access Each slice has 2 FFs and 2 LUTs (COURTESY XILINX, INC.)
FPGA Families extend Architecture ❏ Devices are built, with more capability, but around the same basic architecture ❏ Some additional capabilities ◆ Low voltage versions ◆ Faster clock rates ◆ Different packaging options (Courtesy Xilinx, Inc.)
The need for more stuff ❏ CompEs cannot design on logic, routing, I/O alone ❏ Extreme case from early 90s ◆ 16 port ATM switch, designed on a single board FPGAs (XC3Ks) FIFO memory chips ◆ Design is limited by I/O to memory chips--bring them on-chip 37
Other “Stuff” ❏ Clock managers ◆ Global clock buffering, distribution ◆ Digital Clock Manager (DCM): eliminate skew, phase shifts, multiply or divide clock ❏ Memory ◆ Block RAM ◆ Distributed RAM (repurposed LUTs) ❏ Shift Registers ❏ Dedicated Multiplexers ❏ Carry Look-Ahead Generators ❏ I/O Blocks ◆ SelectIO supports 18 standards (single, differential, various voltage levels, ....) ❏ Embedded Multipliers 38
Hard Macros Hard macros Block RAMs Multipliers CPUs DSPs Soft macros HDL IP Blocks
Block RAMs Distributed RAM Use LUTs as memories Low density Poor performance Block RAM Large-ish dedicated memory blocks Xilinx BRAMs = 18Kb Some configurability Dual-port Data width / depth FIFO, CAM, etc.
Multipliers 18x18 signed 2’s -complement multiplier Two 18b inputs One 36b output 18b enough for many DSP applications Can gang multiple units together for wider data Faster and lower power than multiplier from CLBs
CPUs – PowerPC 405 XC2VP30 has 2 Embedded PowerPC 405 cores Embedded L1 I and D caches No FPU
CPU Connectivity: PLB and OPB IBM Core Connect Processor Local Bus (PLB) - fast on-chip communication On-Chip Peripheral Bus (OPB) - optimized for periphs. (UART, etc) Device Control Register bus (DCR) - used to send and set config.
CPU Connectivity: PLB and OPB (cont.)
CPU Connectivity: OCM On-Chip Memory controller CPU block RAM 2 OCMs – I and D Direct, fast interface Can use dual-port BRAMs for producer-consumer link to FPGA fabric
CPU Links A lot more details on the embedded CPU http://www.xilinx.com/bvdocs/userguides/ppc_ref_guide.pdf http://direct.xilinx.com/bvdocs/userguides/ug018.pdf http://www- 3.ibm.com/chips/techlib/techlib.nsf/productfamilies/CoreConnect_ Bus_Architecture 18-545: FALL 2016 46
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