ASIC and FPGA : inferring multiplier

[Pasacco]

I hope this general discussion is scope of this group.

In my experience, when we describe in VHDL:

c = a * b;

In FPGA design flow, multiplier is automatically "inferred",

while (after synthesis) we can know "what multiplier (for example,
booth multiplier)" and "how many multipliers" are instantiated.

As ASIC novice, today I tried ASIC flow (using Synopsys design
compiler).

After synthesis, netlist SEEMS to be correctly generated.

while I have no idea "what multiplier" and "how many multipliers" are
correctly inferred.

My question is that

1) In typical ASIC flow, should I build the multiplier module MYSELF?

2) How can we know "what multiplier" and "how good the inferred
multiplier" are ?

 

[HT-Lab]

I hope this general discussion is scope of this group.

In my experience, when we describe in VHDL:

c = a * b;

In FPGA design flow, multiplier is automatically "inferred",

while (after synthesis) we can know "what multiplier (for example,
booth multiplier)" and "how many multipliers" are instantiated.

As ASIC novice, today I tried ASIC flow (using Synopsys design
compiler).

After synthesis, netlist SEEMS to be correctly generated.

while I have no idea "what multiplier" and "how many multipliers" are
correctly inferred.

My question is that

1) In typical ASIC flow, should I build the multiplier module MYSELF?

You can build it yourself or instantiate one from the Synopsys Designware
library.

2) How can we know "what multiplier" and "how good the inferred
multiplier" are ?

Without specifying one I suspect that your multiplier is a black box, speak
to your Synopsys FAE,

Hans
www.ht-lab.com

 

[jeppe]

1) In typical ASIC flow, should I build the multiplier module MYSELF?

2) How can we know "what multiplier" and "how good the inferred
multiplier" are ?

Hi

1)
A interfered multiplier will always depend on the type of FPGA your using - XiLinx Spartan 3 would give you 18x18bit=36bit multipliers.
No your dont have to but if can you get some hints here:
http://www.jjmk.dk/MMMI/Lessons/06_Arithmetics/No3_Multipliers/Index.htm

2)
By setting up userconstrains should it be possible to "select" any of the internal multipliers - but it will normally be better if you let the tools do the selection.

Jeppe

 

[Kim Enkovaara]

As ASIC novice, today I tried ASIC flow (using Synopsys design
compiler).

After synthesis, netlist SEEMS to be correctly generated.

while I have no idea "what multiplier" and "how many multipliers" are
correctly inferred.

The selections that DC made are in the synthesis logs.

My question is that

1) In typical ASIC flow, should I build the multiplier module MYSELF?

Usually not, unless you really know what you are doing. The synthesizer
will build a good multiplier if you have the needed licenses.

2) How can we know "what multiplier" and "how good the inferred
multiplier" are ?

What you need is Designware library on top of DC. DC alone can't do good
arithmetic blocks without DW. When DW is enabled in the flow your
synthesis constraints help DC to choose correct multiplier or
you can also force what multiplier architecture is used (see
the manuals).

--Kim

 

[Pasacco]

In the log file,

Processing 'TOP_DW01_add_0'
Processing 'TOP_DW01_add_1'
Processing 'TOP_DW02_mult_0'
Processing 'TOP_DW01_add_2'
Processing 'TOP_DW01_sub_0'
Processing 'TOP_DW02_mult_1'

Some adders and multipliers are inferred.
The design is 120 MHz in FPGA and 450MHz in ASIC.
So, it seems the flow is okay. thank you very much for reply.

 

[Kim Enkovaara]

In the log file,

..
Processing 'TOP_DW02_mult_0'
...
Some adders and multipliers are inferred.
The design is 120 MHz in FPGA and 450MHz in ASIC.
So, it seems the flow is okay. thank you very much for reply.

You should also see somewhere what multiplier architecture was
really selected. If DC+DW are recent versions it should be
either csa (Cassy-Save array) that does not need DW license
or pparch (Delay-optimized Booth Wallace) which needs DW
license.

You need something like this for DW in the synthesis scripts

set synthetic_library [list dw_foundation.sldb]
set link_library [concat $target_library $synthetic_library]
set search_path [concat $search_path [list \
[format “%s%s” $synopsys_root “/dw/sim_ver”]]]
set synlib_wait_for_design_license [list “DesignWare”]

and to force faster multiplier (if the constraints are not
finished) something like this.

set_implementation pparch [list my_multiplier]


There are also many different adder architectures available
(rpl,cla,pparch,clf,bk,csm,rpcs at least). So the frequency
you achieved could be much higer with some synthesis script
changes.

--Kim

 

[Pasacco]

Hello Kim, I have problem to use components in the design ware.
Let me ask one question

I wish to use the "DW_sqrt_pipe" component (square root).

In my VHDL, I simply instantiated the component:
--------------------------------------
.....
U1 : DW_sqrt_pipe
generic map (width => 32,
tc_mode => 0,
num_stages => 2,
stall_mode => 1,
rst_mode => 1 )
port map (clk => clock,
rst_n => reset,
en => en,
a => a1,
root => root1 );
--------------------------------------

The synthesis log contains warning:
--------------------------------------
Warning: Can't find the design 'DW_sqrt_pipe'
in the library 'WORK'. (LBR-1)
Warning: Unable to resolve reference 'DW_sqrt_pipe' in 'TOP'. (LINK-5)
--------------------------------------

It seems that the library "DW_sqrt_pipe" is not compiled into WORK.

My script is following:
--------------------------------------
set synthetic_library /cad/synopsys/libraries/syn/dw_foundation.sldb
set target_library /opt/data/Syn_libs/tcbn90gthptc.db
set link_library /opt/data/Syn_libs/tcbn90gthptc.db
analyze -format vhdl {SQ.vhd}
elaborate TOP
......

 

[Kim Enkovaara]

Could you please guide me what needs to be modified in the script?
Thnak you in advance.

You could copy settings that were in my last mail, that example was from
DW documentation. I think you need dw also in the link library setting.
It has been a while since I used DC, one option would be also to use
the DC reference flow scripts that can be downloaded from the solvnet.
It has all the bells and whistles in place for different DC versions.

--Kim

 

[Pasacco]

hi Kim. i will investigate. thank you for reply.