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T >= Tcq + Tcomb + Tsetup - Tskew

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kunal1514

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effects of hold time violation

Hi All,

Can any body tell me

1) What causes HOLD VIOLATIONS in DESIGN.

2) How it effects DESIGN.

3) What changes need to be done to make DESIGN work.
 

sunilbudumuru

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thold = tskew

Hi Friend,

1) What causes HOLD VIOLATIONS in DESIGN.
Simply, data should be hold for some time (hold time) after the edge of the clock. So, if the data changes with the hold time might cause violation. In general, hold time will be fixed during backend work (during PNR) while building clock tree. If u r a frontend designer, concentrate on fixing setup time violations rather than hold violations.

2) How it effects DESIGN.
If a chip is done with some setup violations it can work by reducing the frequency.
If achip is done with hold violations, JUST DUMP the chip. This is how it effects at the end of the day. Hold vilations needs to be fixed.

3) What changes need to be done to make DESIGN work.
PNR tools will route and place the cells in such a way that no timing violations will occur. If still u face hold violations, u can manully work on it to fix. Manually place the cells to avoid hold violations, or in the worst case, u can keep some buffers in the datapath to avoid hold violations (but be sure setup timing is not effected.)

Hope this answered your questions.

Thanks & Regards,
Sunil Budumuru.
 

cnlionking

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hold time timing ff corner

hi sunilbudumuru ,
you said "If achip is done with hold violations, JUST DUMP the chip. "
why can't reducing the frequency to settle the hold violation as setup violation ?
could you explain it clearer ?
thans you .
 

ljxpjpjljx

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how to eliminate hold violation

because the hold time has some releationship with the setup time , so if you reduce the frequency , it will make the hold time violation more ill!
 

koggestone

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hold time violation effects

>you said "If achip is done with hold violations, JUST DUMP the chip. "
>why can't reducing the frequency to settle the hold violation as setup violation ?
>could you explain it clearer ?

Equation for Setup Time
Tclk > Tclktoq + Tlogic + Tsetup + Tskew + Tjitter

Equation for Hold Time
Tclktoq + Tlogic - Tskew > Thold

Note that Hold Time equation is independent of clk frequency(i.e Time period Tclk)

key things to note from above equations
a) once the silicon comes back , if u have setup time problem , u can
increase the clock period (Tclk) to fix it , whereas if u have hold
time problem , its a more serious problem and u will need a new
metal fix tapeout . ( But u can still test the current chip using Low supply voltage,
or High temperature or SS corner part that decrease hold time violation)
 

sunilbudumuru

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hold violations fix increasing temperture

Hi Friends,

Pls. make a note that HOLD violations are dangerous than SETUP. To keep it simple way, SETUP timing depends on the frequency of operation. But HOLD time is not. Let us see the equations here.

T = Frequency of operation (can be variable)
Tcq = Flop clock to Flop output delay (fixed/constant)
Tcomb = Delay od the combinational logic between the Flops (can be variable)
Tsetup = Setup time of a Flop (fixed/constant)
Thold = Hold time of a Flop (fixed/constant)
Tskew = Delay between clock edges of two adjacent flops (delay offered by clock path) (can be variable)

For SETUP,
T >= Tcq + Tcomb + Tsetup - Tskew

If you have setup time means u r violating the above rule. i.e some how the equation becomes
T < Tcq + Tcomb + Tsetup - Tskew

Now let us consider two cases.

Case1: During the Design development phase itself.

Now, you have three variables (T, Tcomb, Tskew.) to avoid the setup violation.
T : Reduce the frequency such that u saticify T >= Tcq + Tcomb + Tsetup - Tskew. But do u think it is the correct solution. Obviously, NO. This is because we have other options to avoid setup violations right.
Tcomb : If you reduce the combinational delay (between the Flops of violated path) such a way that T < Tcq + Tcomb + Tsetup - Tskew will become T >= Tcq + Tcomb + Tsetup - Tskew. So, the SETUP violation is avoided. How do u reduce the combinational delay??? Try different logic structure without effecting the functionality. or try to reduce the more fanout nets within the logic. Or upsize or downsize the cells. If it worked out thats fine.
Tskew: If u increase the skew, u can change T < Tcq + Tcomb + Tsetup - Tskew to T >= Tcq + Tcomb + Tsetup - Tskew. How to increase the Tskew? Just keep buffers in the clock path. But be sure doesnt effect the HOLD timing.

Case2: After the CHIP is manufatured and is in your hand.

In this case, one cannot access the Tcomb and Tskew. Only the variable that can handle is T.
So, Just reduce the frequency (T) such that the violated equation, T < Tcq + Tcomb + Tsetup - Tskew becomes violation free equation T >= Tcq + Tcomb + Tsetup - Tskew.

So, if u have setup violations on a manufatured chip, u can make it work by reducing the frequency.

For HOLD,
Thold + Tskew <= Tcq + Tcomb

If you have setup time means u r violating the above rule. i.e some how the equation becomes
Thold + Tskew > Tcq + Tcomb and ur aim is to make Thold + Tskew <= Tcq + Tcomb

Now let us consider two cases.

Case1: During the Design development phase itself.

You have two variables in hand (Tcomb, Tskew) to avoid HOLD violations.
Tcomb: Increase the Tcomb by adding buffers in the data path. Thus u can change the situation from Thold + Tskew > Tcq + Tcomb to Thold + Tskew <= Tcq + Tcomb. But this might effect the SETUP time as you are increasing the delay of combinational path. So this may not be the perfect solution always.

Tskew : Reduce the clock skew so that you will land on Thold + Tskew <= Tcq + Tcomb. To reduce the clock skew, the best solution is to take the help of your PNR engineer.

Case2: After the CHIP is manufatured and is in your hand.

Do you see any variables that will fix the hold violations after manufaturing?????!!!!!! NO right. So, its time to DUMP the chip as we dont deliver malfunctioning chips to the customers.

So becareful with the HOLD violations.

Note: One can get those equations if u put the those scenarios on a paper and develop the timing diagrams.

Hope I've explained it properly.

Regards,
Sunil Budumuru.

Added after 46 minutes:

Hi koggestone,

It is nice information. Could you please give us more information on

" u will need a new metal fix tapeout . ( But u can still test the current chip using Low supply voltage, or High temperature or SS corner part that decrease hold time violation)"

It may give us more information on this topic.

Thanks in advance.

Regards,
Sunil Budumuru.
 

koggestone

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hold time violation high voltage

> Hi koggestone,
> It is nice information. Could you please give us more information on
> " u will need a new metal fix tapeout . ( But u can still test the current chip using
> Low supply voltage, or High temperature or SS corner part that decrease hold time >violation)"

what i meant was , when u have hold time violation , u dont need to throw away chip and wait for 3 months for fixed chip to come back. in the meanwhile , by playing with voltage and temperature , u can do other functional tests on the chip . since normally hold time simulation are done at " FF corner , high voltage, low temperature " which is the pessimistic case for hold time , by decreasing voltage , using high temperature, and a SS corner chip , we may be lucky enough to find a part that works , to do other functional tests to catch any other bugs before next tapeout.
 

cnlionking

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hold time and setup time corners

Thanks every one above , and especially to sunilbudumuru, your explaination is very clear ,that's really great for me !
and I get it now .
To put it simply, when the clk edge arrived , the signal on the D(input of dff) shouldn't change earlier than hold time end . that is, the new signal from the previous level FF shouldn't arrive on the D port before the end of hold time .
so there's require :
Tcq + Tcomb >= Thold + Tskew .
and this constraint is for two adjacent FF on the same clk edge , so the hold time have no relationship with clk freqency .
thank you again . :)
 

sunilbudumuru

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what are hold violations

Hi koggestone,

Thanks for the valuable information.
So there are some other uncertain procedures that will try helping to fix hold violations.

By the way,don't take the DUMP THE CHIP in true sense. No one is ready to DUMP the chip after putting lots of money on that. Ofcourse, there will be test chips and MPWs, Revisions stuffs like that in which the critical issues will be fixed.

What I was trying to highlight is do not take the HOLD violations easy. They may worsen the situation.

:)

Regards,
Sunil Budumuru

Added after 19 minutes:

Hi cnlionking,

U r correct,
Hold check for two adjacent FFs will be on the same clk edge , and no frequency comes into picture.
But Setup check will verifiy that the data path delay is small enough so that the data launched from first FF reaches next FF within one clock cycle. And here clock cycle (frequency) comes into picture.

It is nice to hear that you got the required from our discussions.
 
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