( ESNUG 287 Item 2 ) ---------------------------------------------- [4/30/98]
Subject: ( ESNUG 286 #11 ) Seeking Many Decimal Digits Delays On Inputs
> I was wondering if anyone knows an easy way to automatically set the drive
> at each pin such that I am assured that there is, say, 0.5ns at all inputs.
> The set_drive command will work, but I need to know the capacitance at
> each input port in order to set each pin's drive correctly, e.g. set_drive
> <0.5ns/capacitance> <input_port>. The problem is knowing each input's
> capacitance. I've tried to get the capacitance attribute by using the
> 'get_attribute' command, unsuccessfully. I finally wrote a Perl script
> to create a set of set_drive commands, based on a report using the
> 'report_net' command. The problem with that is, the resolution is only
> to 2 decimal places (load field):
>
> Net Fanout Fanin Load Resistance Pins
> ------------------------------------------------------------
> INPUT_1 1 1 0.01 0.00 2
> INPUT_2 1 1 0.01 0.00 2
>
> "Load" here may be 0.0149 or 0.0101 - so you don't generate a very precise
> drive value this way. I don't want to change my lib input capacitances
> from pF to fF right now. That would certainly help in the report. Any
> other ideas?
>
> - Chuck Bellman
> Motorola NCSG SemiCustom
From: "Russell Ray" <rray@msai.mea.com>
John,
Since he already mentioned that he used the get_attribute command, I'm not
sure if he has tried this, he probably has.
I have had to use this for other things, too, and have found it to be a
pain. But, the only way to get all the digits in the library file is to
either have the library file (which most of us can't get) or use the
get_attribute command.
I have used the report_lib, get_attribute, and perl to create the scripts I
wanted. The report_lib piped to a file so that I would have a list of all
the library cells (sounds like you already know these). Then I use perl to
create a dc script that would run get_attribute on each of the cells and
their input pins. All dc_shell output goes to a file. Then I use another
perl script to process the output of dc_shell to create my script (in his
case, the set_drive script).
It is all very messy and time consuming, but it does work.
- Russell Ray
Mitsubishi Semiconductor Durham, NC
---- ---- ---- ---- ---- ---- ----
From: Scott Evans <scott@NPLab.Com>
John,
You may want to try report_net -verbose -connections. An example:
Connections for net 'ck$ck_gen$n1531':
pin capacitance: 0.063
wire capacitance: 0.170708
total capacitance: 0.233708
wire resistance: 0.101727
number of drivers: 1
number of loads: 5
number of pins: 6
Driver Pins Type Pin Cap
------------ ---------------- --------
ck$ck_gen$U427/O Output Pin (INV1L) 0
Load Pins Type Pin Cap
------------ ---------------- --------
ck$ck_gen$count4_reg_4/SEL Input Pin (DFZN) 0.0126
ck$ck_gen$count4_reg_3/SEL Input Pin (DFZN) 0.0126
ck$ck_gen$count4_reg_2/SEL Input Pin (DFZN) 0.0126
ck$ck_gen$count4_reg_1/SEL Input Pin (DFZN) 0.0126
ck$ck_gen$count4_reg_0/SEL Input Pin (DFZN) 0.0126
It provides a lot more detail.
- Scott Evans
NeoParadigm Labs San Jose, CA
---- ---- ---- ---- ---- ---- ----
From: David Cassetti <david.cassetti@tempe.vlsi.com>
John,
You might want to read Solvit article synth-022860.html. The gist of it
is, you can do something like:
set load 100000 your_input_port
set drive .000005 your_input_port
This will give you a delay of 0.5 ns
- David Cassetti
VLSI Technology Tempe, AZ
---- ---- ---- ---- ---- ---- ----
From: Paul Fletcher <paulf@chdasic.sps.mot.com>
John,
Give this a try, I have done some of this type of work to add constraints
to an existing library to get better synthesis results, so I new how to
get these values.
T_L = cdr1synPwcshV300T125
remove_varilable capacitance_attribute > /dev/null
wild = "*" /* used so wild carding does not look like a "C" comment */
_DRIVE = "1" /* all 1 x drive cells */
cap_attr = 0.0 /* make sure this is a float */
foreach(cell_name, find(cell,T_L + "/" + wild + _DRIVE) {
pinlst = filter(find(pin,cell_name + "/" + wild), "@port_direction ==in") > /dev/null
echo cell_name pinlst
foreach(pinn,pinlst) {
list pinn
cap_attr = get_attribute(find(pin,cell_name + "/" + pinn) capacitance)
list cap_attr
}
}
You can remove the echo and list commands and add commands to use the data
this returns. Here is an example of what the script returns:
cdr1synPwcshV300T125/sdffps_1 {sb, d, ck, sdi, se}
pinn = "sb"
Performing get_attribute on port 'sb'.
capacitance_attribute = {0.015935}
pinn = "d"
Performing get_attribute on port 'd'.
capacitance_attribute = {0.010037}
pinn = "ck"
Performing get_attribute on port 'ck'.
capacitance_attribute = {0.006111}
pinn = "sdi"
Performing get_attribute on port 'sdi'.
capacitance_attribute = {0.007052}
pinn = "se"
Performing get_attribute on port 'se'.
capacitance_attribute = {0.010471}
Hope this helps.
- Paul Fletcher
Motorola Chandler, AZ
---- ---- ---- ---- ---- ---- ----
From: "Stickel, Tedd K" <Tedd.Stickel@unisys.com>
John -- to characterize library timing I've used a large set_load on
all inputs, say 5000pF, and then a small set_drive, say .0001 on those
same inputs. The large set_load washes out any intrinsic pin C.
- Tedd Stickel
Unisys
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