RTL_compiler SDF

Hi giohdl,
The write_sdf is an unsupported command, but it should work to give you a basic SDF. The syntax is:

write_sdf > filename

There's another way to get SDF is to use BuildGates (bgx_shell) from RC package. The basic script is:

invoke BuildGates: bgx_shell

read_dotlib your_library
read_verilog -st your_netlist
write_sdf file_name

Thank you synthman,

I'll try with BuildGates because
I need to specify some option
in the SDF file.

Hi Giohdl,

Another way is to simulate without sdf (i.e. 0 delay, unit delay etc). Usually, this is enough. What is the use of timing before P&R?

Regards,
Eng Han

Hi EngHan,

I think you're right in general.
My circuit contains several interconnected FSMs so
I've preferred to simulate before P&R
avoiding zero delays in cells.

Regards,
Giohdl

Unit delay can lead to you spending time debugging issues that don't exist. Avoid like Martha Stewart's stock tips ;-)

Since you can't really build an SDF that will realistically reflect your final timing atleast until CTS is done, then here are two alternate solutions that I've used but no-one has suggested thus far:

1) Create and compile a modified version of your verilog/vhdl gate library with 1nS clk->q (or qn etc) on flops (and latches), then use zero delay for combinatorial cells. Design will now function accurately pre-layout.

2) Alternately why not create a dummy SDF that attaches delay values to specific cell types rather than specific instances? You can do this quite simply. The following example will annotate a 1ns delay to all flops in the design that are of type "floppy"...

(DELAYFILE
(SDFVERSION "2.1")
(DESIGN "dodgy")
(DATE "17-May-2006")
(VENDOR "Shoddy Devices Inc")
(PROGRAM "Manually created. Annotated all DFFs with CP->Q of 1:1:1")
(VERSION "1")
(DIVIDER /)
(VOLTAGE 1.65:1.65:1.65)
(PROCESS "1.5:1.5:1.5")
(TEMPERATURE 125:125:125)
(TIMESCALE 1 ns)
(CELL
(CELLTYPE "floppy" )
(INSTANCE * )
(DELAY
(ABSOLUTE
(IOPATH CP Q (1:1:1) (1:1:1))
)
)
)
)

At "Shoddy Devices Inc", we've used both methods successfully. However the second method might be simulator depedent. I'd recommend the first method (unless your vendor doesn't give you simulator library source code).

HTH

Crispy Duck

Hi Crispy Duck,

Your reply is enlightening. Actually, I am going along your approach but my reply was short... the effort to get gate level simulate correctly depend on many things, so it is hard to generalise.

What I did is to change the delay directly in the verilog model (std cell, and sometime even IP like memory), those numerbs in the "specify". This does not require additional file, and the simulation run faster and less memory. The delays are as what you suggest; sometime I use a very small delay for clock buffer, and use the actual delay for delay cell.

To be more complete, typically we also have to tune the testbench, and sometime adjust the cycle where the data will start to appear. Also have to disable the hold/setup/etc of the synchonisers so that the control-logic does not end up with all "X". Depend on the library and the version of the software you use, you have to patch the SDF to make it "annotatable" (for example you have posedge in .lib, but not in the verilog model). In my last project, I have to use a patched version of Encounter to generate the sdf and 5.* cannot (I might remember wrongly here, but just get the idea) with setuphold / recrem (so that -ve check is supported). Lastly, remember to check the timescale has sufficient accuracy. I think this list can add on; there is always something new in every project.

Regards,
Eng Han

I think this list can add on; there is always something new in every project

 Hi EngHan,

I read your comment saying that you solved some SDF annotation problems using an Encounter patch. Kindly share the solution if the problem was same as below:

 When I try to do an sdf annotation in Cadence I get lots of warnings like:

ncelab: *W,SDFGENNF: Generic "TPD_ip1_op_posedge" not found in component "test.dff_1.g16:" <./gen_test1.sdf, line 20>.

The generic "TPD_ip1_op" is present in the library vhdl model file but the _posedge generic is not present.
 

Also, I use the write_sdf command in Encounter after P&R and CTS to generate the SDF file, is that correct?