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What is the practical implication of negative setup and negative hold?
How do simulators handle negative timing checks?

prem_eda wrote:

> What is the practical implication of negative setup and negative hold?
> How do simulators handle negative timing checks?

Setup and hold together just define the time during which the input signal
should not change in order to get a stable output. Having either a negative
setup or a negative hold (both at the same time is very unlikely just
places that stable period before (negative hold) or after (negative setup)
the clock edge i.s.o. around it. I see no reason why a simulator would have
any problems with this.

Regards,

Pieter Hulshoff

Whenever we perform Timing simulation, the SDF file contains Timing check
tasks (For example $setuphold) constructs which has some negative
values.Normally, the simulator has a negative timing constraint algorithm
to handle the -ve values in the timing check tasks and in some cases I see
that being rounded off to zero.
So,any one you guys have faced this? Why is the simulator doin this?

prem_eda wrote:
> Whenever we perform Timing simulation, the SDF file contains Timing check
> tasks (For example $setuphold) constructs which has some negative
> values.Normally, the simulator has a negative timing constraint algorithm
> to handle the -ve values in the timing check tasks and in some cases I see
> that being rounded off to zero.
> So,any one you guys have faced this? Why is the simulator doin this?

Hmm, a tad more information would be nice.
1. Which simulator are you running?
2. How much is th negative timing constraint?
3. What is your simulation resolution?

One reason I could think of is that the negative timing constraint is
smaller than your simulation resolution, but perhaps other reasons may pop
up if you can give a few more specifics. You say 'in some cases'; I presume
that means that sometimes it's handled correctly?

Regards,

Pieter Hulshoff

I am using Modelsim SE version 5.8c on Windows 2000 platform.The simulator
resolution is in ns and the SDF file has a a ps resolution.I do not have
the SDF file with me But I can just quote an example which is something
like this.
$setuphold( posedge s1clk, negedge wr_en1, 180, -74,
notifier,,,ds1clk,dd);

prem_eda wrote:
> I am using Modelsim SE version 5.8c on Windows 2000 platform.The simulator
> resolution is in ns and the SDF file has a a ps resolution.I do not have
> the SDF file with me But I can just quote an example which is something
> like this.
> $setuphold( posedge s1clk, negedge wr_en1, 180, -74,
> notifier,,,ds1clk,dd);

Are you sure your simulator uses a resolution of 1 ns? If it is, then any
time setting will be chopped down to a number x 1 ns, e.g. 3.9 ns => 3 ns,
1.1 ns => 1 ns, 150 ps => 0 ns. You might want to see what results you get
if you set your simulation resolution to 10 ps or 1 ps.

Regards,

Pieter Hulshoff