bgs: whats diff bet set_max_capacitance and set_load?
i dont know ans
me: set_max_capacitance applies on pin not on pin and net
but set_load applies capacitance constraint on both net and pin
i.e whole output port gets loaded
bgs: max cap of cell (pin) will be precalculated by lib vender whats the use of setting it again
for higher value
me: pin cap can vary based on fanout...using which u can control fanout
bgs: what?
me: ultimately u r trying to control fanout==>drive strength==>delay
load cap==Cpin+Cnet
either u try to model Cpin or Cnet or both together
as u know gate delay=function of (input transition, Cload)
where Cload=Cpin+Cnet
hence we have set_max_cap and set_load
bgs: 1 sec ....for example if i have and (a,b,y) im seting max cap for y as 2. but default value of y in lib is 1 will it violate
*andgate(a,b,y)
sud i keep set_max _cap on y
me: yes....then tool tries to find a cell that meets your constraint
bgs: oh yes its doing
thanku
one more thing doubt pls
one more doubt pls
me: no need to keep set_max_cap for each and every cell...its impossible...
try to provide a moderate value so that tool take care of high fanout net synthesis
bgs: what is diff between timing engine and timing engine PT
me: DC target is to synthesize the code efficiently.....PT works exclusively on timing analysis
DC-uses less accurate timing algorithm
PT -uses Arnoldi algorithm which is synopsys patented and considered to be very accurate
PT timing correllation with SPICE results are really great
DC-no netlist editing facility
PT can do netlist editing
PT can do crosstalk analysis DC cant
PT can do variation analysis DC cant
bgs: get_allternate_cell is available in DC
me: ya.... u can try to find altrenative cells
thats all
but ofcourse DC can do incremental compilation
bgs: in DC flow if timing is not met will they change constraints or its fixed
me: more important is with PT there are several designs successfully taped out !!!!!
u have two options... improve ur HDL code....if still doesnt work..u have to change constraints itself ..but this is last option
bgs: if constaints r not realistic?
me: what u mean by realistic?
bgs: for example u have clock period is 3 inputdelay is 2
me: input and out delays are always exceptional cases and those can controlled depending on external interfaces...
bgs: ya ur ri8
me: bgs..... let me take a cup of coffeeee
bgs: but timinig is not met
me: i will come back later
bgs: k carry on

18 minutes

me: u can shoot again.....but timinig is not met.....wht is the question

19 minutes

me: gayab???
bgs: ya im back
my question was do i have n option of changing constraints if its not met
as synthesis engineer what all constraints i can change
?
me: welll.... frankly speaking we cant change constraints at logic synthesis stage (physical synthesis onwards we have a little control on constraints !!)
with the available clock if u cant meet timing even after improvement in code, logic and data path optimizations in DC then the left option is to increase clock period itself !
bgs: do u mean to say clk source, n/w latency, uncertainity will given as spec
me: as a part of spec u wont get latency etc....u will get only clock frequency...thats what u have to cahnge finally
bgs: ya then can i play with uncertainity , input n output dely?
max n min delay
me: to certain extent.... provided that if u have back annotated info of similar designs implemented in previous designs....
but at first itself u have modeled everything based on previous design experiences...
then u cant relax those constraints further
bgs: what about drc constraints? can i change them
me: u can provided if u dont have problem with timing and power and reliability !
bgs: k
me: becuase ur timing may be ok but there may be drc violations...in that case power consumption can be more
bgs: so if can i change constraints(not clk period
) n met timing
me: and also imagine a person can carry 50kg... then he can go for 5 kms... now imagine same person is forced to carry 100 kg...can he walk 5kms (reliability issue)?
bgs: hm
me: if u have modeled ur constraints properly then there is no room to change it...
but if u have over constrained the design at first then u got violation...in that case u can relax constraints
bgs: k
so in DC we play with constaints n fix them
me: no..we play with data path optimizations and logic optimizations to achieve timing...
we play with low vt cells and high vt cells to meet timing and power
bgs: hey that we do in
PT
me: but remember...PT is mainly used for analysis
PT wont do any optimization by itself
bgs: k, in dc we make sure violation is clear then y PT?
me: as i said PT timing engine is more robust.... more accurate compared to DC..hence get clearance from PT as well
bgs: ok
one more doubt yar in pt im geting some violation what all option i have other than swap cell to clear violation
me: size the cell
before sizing try to see bottleneck cost
see the depth of data path...if it is large then probably it is better to send the netlist back to synthesis stage
for data path optimizations
bgs: size the cell is increasing drive strength only na
r some thing else?
me: u can increase or decrease...depending on the delay offered by the cell...
also u can add buffers just before a cell which is causing larger delay..
also recalculate path delay with true path analysis...
this u can get with path inspector
bgs: how to add buffer i have not tryed that option
what does recalculate path delay with true path analysis... do?
me: get_recalculated_timing_paths -from xyz -to abc
insert_buffer/remove_buffer
bgs: what does this do buy recalculating timing path?
me: u have to provide buffer name and cell name
bgs: by*
when will we use get_recalculated_timing_paths -from xyz -to abc
me: if u have small violations and less number of violations then use recalcu***
bgs: k
thanku so much
:)
me: what happens in general is while calculating delays worst delay corresponding to different timing arc is considered
hence timing analysis becomes pessimistic....
to make it more accurate tell the tool to calculate timing based on actual true timing arcs
when u do this u may not get any violation...in that case u can tape out !!!!!
hey bgs ....i am leaving now.... see u tomorrow for any more questions.....
hope u r happy now with so many cleared doubts..
bgs: hey thanks yar u gave ur valuable time!!!
ya im :-

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