Synthesizing SRAM
timing and Periphery
using Synopsis
Robust
Low
Power
VLSI
By: Jim Boley
Previous work
 Bitcell array layout complete
 DRC clean, LVS…not working
 Timing and decoder simulated and
synthesized
53.84μm x 15.12μm = 814.1μm2
2
Timing/Periphery Synthesis
}
BL Drivers
}
BLB Drivers
<- Decoder output
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
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
Converting Layout to a
Milkyway Library
6
Step 1
 Two options for
exporting the layout:
 GDS (Stream)
 LEF
Step 2
 Create new milkyway library
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
Step 4
 Create the FRAM view, used for place and
route
 For macros use Cell->Make Macro Abstract
Step 4
 Extract blockageallows you to
completely block
certain routing
layers or keep all
blockage
information
 Extract pin by texteasier if text layer
matches pin layer
Final Steps
 Set PR Boundary
 Cell Library->Set PR Boundary
 Define unit tile wire tracks
 Wire Tracks-> Define Unit Tile Wire Tracks
Run ICC Place and Route
13
Create Top Level Verilog
module 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
Reference the Milkyway Library
Definitions.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
\
\
\
\
Load setup.tcl & definitions.tcl
 Tool will complain that the bitcell_array does not
have a corresponding logic cell description…this is
okay
Floorplan and power grid created
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
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
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
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)
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
Questions?
23