RobustLowPowerVLSI

RobustLowPowerVLSI

Synthesizing SRAM timing and Periphery using SynopsisBy: Jim Boley

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Previous work

53.84μm x 15.12μm = 814.1μm2

Bitcell array layout complete DRC clean, LVS…not working

Timing and decoder simulated and synthesized

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Timing/Periphery Synthesis

}

}

BL Drivers

BLB Drivers

<- Decoder output

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Constraints file

set_driving_cell -lib_cell NBUFFX16 -pin Z [get_ports clk]set_driving_cell -lib_cell NBUFFX4 -pin Z [get_ports {read enable}]set_load [expr 16 * [load_of saed90nm_typ_ht/INVX2/INP] ] [all_outputs]

set_driving_cell- sets input fan in set_load- sets output fan out

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Final Steps Finish bitcell array- (power grid, add body

contacts, add pins) Convert layout into a Milkyway Library Create final top level verilog netlist Place and route using ICC

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Converting Layout to a Milkyway Library

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Step 1 Two options for

exporting the layout: GDS (Stream) LEF

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Step 2 Create new milkyway library

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Step 3 Import GDS or LEF

Cell Library -> Stream In Cell types: standard cell,

pad cell, filler cell, macro. If blank, default is std cell

Layer file- converts GDS to milkyway. If none, layer names defined in GDS are retained by milkyway

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Step 4 Create the FRAM view, used for place and

route For macros use Cell->Make Macro Abstract

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Step 4 Extract blockage-

allows you to completely block certain routing layers or keep all blockage information

Extract pin by text- easier if text layer matches pin layer

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Final Steps Set PR Boundary

Cell Library->Set PR Boundary

Define unit tile wire tracks Wire Tracks-> Define Unit Tile Wire Tracks

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Run ICC Place and Route

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Create Top Level Verilogmodule top ( adr, read, enable, clk, data_in, BL, BLB, WL, data_out ); input [3:0] adr; input [15:0] data_in; output [15:0] BL; output [15:0] BLB; output [15:0] WL; output [15:0] data_out; input read, enable, clk;

timing_1 tim (.adr(adr), .read(read), .enable(enable), .clk(clk), .data_in(data_in), .BL(BL), .BLB(BLB), .WL(WL), .data_out(data_out) );

bitcell_array bitcells (.WL(WL), .BL(BL), .BLB(BLB));

endmodule

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Reference the Milkyway LibraryDefinitions.tcl:

create_mw_lib -technology ../ref/techfiles/saed90nm_icc_1p9m.tf \-mw_reference_library {../ref/saed90nm_fr bitcell_array} \-hier_separator {/} \-bus_naming_style {[%d]} \-open ./SRAM_TOP

set tlupmax "../ref/tluplus/saed90nm_1p9m_1t_Cmax.tluplus"set tlupmin "../ref/tluplus/saed90nm_1p9m_1t_Cmin.tluplus"set tech2itf "../ref/tluplus/tech2itf.map"

set_tlu_plus_files -max_tluplus $tlupmax \ -min_tluplus $tlupmin \

-tech2itf_map $tech2itf

import_designs -format verilog \ -top top \ -cel top {../source/top.v}

source ../source/constr.sdc

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Load setup.tcl & definitions.tcl

Tool will complain that the bitcell_array does not have a corresponding logic cell description…this is okay

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Floorplan and power grid created

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ERROR! Unable to place bitcell array Unable to get past this error Tried creating milkyway library from LEF file,

but icc was not able to recognize ports

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Periphery placed and routed Was able to place and route periphery cells Final dimensions: 127um x 34 um Original bitcell array size: 53.84μm x 15.12μm

Synthesized periphery 5.3X larger than bitcell array

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Pitch Matching Goal: 1x1 matching of periphery cells to bitcells Vertical pitch of std cell 3x that of bitcell

Solutions: place 3 std cells horizontally Increase pitch of bitcell

Better solution: create standard cells whose pitch match the bitcell

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Conclusions Standard cell library cells are not suited for SRAM

periphery Periphery usually consumes on 20% of total area

because it is pitch matched to the bitcells Place and route tool could be useful for doing top

level routing (assuming placement information could be easily passed to the tool)

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Why I didn’t like Synopsys No online forums for answering

questions (like cadence has) Nothing worked the first time

Spent 70% of my time debugging the tool, some problems could not be overcome

Tech support infrastructure not adequate (i.e. working tutorials, wiki FAQs, students with experience)

Conclusion: changing tools = huge start up cost

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Questions?