[SI-LIST] Re: SA12E Vil and Vih
- From: "Lynne Green" <lgreen@xxxxxxxxxxx>
- To: <rajat.chauhan@xxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 10 Jan 2002 14:02:47 -0500
Hello, Rajat,
There is a lot of information on this in si-list emails in the archive.
So this is just a quick summary to get you started.
Take the sum of your delays:
Sum for the signal path (either one chip to the next, or =
latch-to-latch):
(internal_chips + I/Os + pkgs + tlines + setup/hold times + jitters + =
safety margin)
must be less than max_allowed
Some of these delays may come from simulation, others from datasheets.
Delays are data-dependent; rising and falling delays are usually not =
equal.
Always take the biggest delay for each term. If you can afford to =
simulate
the entire path, your worst-case number may be faster, since there is
some interaction between the various stages (particularly for =
latch-to-latch
simulations).
* max_allowed is set by your maximum signaling rate, signaling delay =
path,
and setup/hold requirements at the receiver.
* If your I/O timing is taken at the core_ref voltage on the core =
interface pins
and at VREF on the pad, you have clean timing references for the driver.
* If your output is timed from the pad at VREF and your receiver pad is =
timed at Vinh/Vinl,
you have clean timing references for the tline delay and package delays. =
If these
are timed from the package pins, you need to add in the package delay =
separately.
* If your chip input (receiver stage) is timed from Vinh/Vinl at the =
input pad to
the chip's core_ref, you have clean timing through the receiver. Note =
that the driving
and receiving chips do not need to have the same core_ref.
* Watch out for chip timing tables taken at some core_ref, while I/O =
timing taken
from some other voltage (from data_core to pad). This can lead to =
timing errors at=20
drivers and/or receivers.
* package delay: needs to include any ringing or settling time effects =
due to package
impedances, which in turn depends on whether on-pkg and/or on-die =
terminations are=20
used.
* jitter: how much jitter is tolerated (or expected) depends on things =
like the=20
bit-error-rate you need and the circuit design (especially the PLL). =
While jitter
never has a "max" value (spikes will occur), it does have predictable =
characteristics.
The safety margin addresses all uncertainty in the accuracy of your =
numbers.
How much of a safety margin to leave, and how to adjust it for what you =
know is=20
unknown, is an engineering design art. What we used 20 years ago is =
useless
for today's designs. The only thing I can safely say is a zero for =
safety margin
means there is a risk the design could work intermittently, unless all =
of your other
numbers are safely conservative.
The best trick I ever learned was to draw a "timing diagram" showing the =
transitions,
and marking on each transition where the timing was measured for the =
incoming and
outgoing signals. This makes it easy to see where there are mismatches =
in the
timing definitions. do one for riding and one for falling, to keep the =
picture from getting
too cluttered.
Best regards,
Lynne
-----Original Message-----
From: rajat.chauhan@xxxxxx [mailto:rajat.chauhan@xxxxxx]
Sent: Wednesday, January 09, 2002 10:00 PM
To: Lynne Green
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: SA12E Vil and Vih
Hello Mr. Lynne,
I use to face a lot of problem in specifying timings of my design. =
First
please tell me what do mean by "habit of leaving a 'safety=3D20' margin =
("padding")
in timing allowances".
Second I can't able to make out when to specify my delays in case of =
output
buffers, simulating my buffer alone without considering package =
parasitic, or
considering package parasitics, or switching 3-4 or more similar buffers =
having
same pow-gnd connections simultaneously, or switching 3-4 or more =
similar buffers
having same pow-gnd connections in some other pattern. Because in each =
of the
case i find different delays. Is it good to give delay range or worst =
delays or
any other?=20
thankyou
Rajat
>=20
> My I/O design experience was that designing 3.3V I/O over all
> process/temperature/power supply variations did indeed lead to
> receiver logic thresholds (even at DC) that were worst-case=3D20
> within ~20-30 mV of Vinh and Vinl.
>=20
> Back in the good old days (about 2 years ago), the "delay" used
> for chip timing analysis for I/O circuits could be assigned to =
the=3D20
> "board" or the "chip" timing (and novice designers always assumed
> it was done in the other one). To make matters more interesting,
> board designers and I/O designers (and I/O standard cell designers)
> can (and often do) use different reference voltages for their timing
> simulations, leading to increased timing uncertainty.
>=20
> Back then, experienced designers had the habit of leaving a =
safety=3D20
> margin ("padding") in timing allowances. Unfortunately, today the
> total clock cycle is >10x faster than our old rule-of-thumb padding.
> Sigh...
>=20
> - Lynne
>=20
> "All the world's an analog stage, whereon digital plays bit parts."
>=20
>=20
> Andrew.Ingraham@xxxxxxxxxx> wrote:
> There are various reasons why a different edge rate causes the timings
> to change. One is that the input buffer has some unpredictable input
> offset voltage, so the actual "switch point" might be anywhere between
> Vil and Vih (an oversimplification, perhaps, but let's use it anyway).
> Another is gain bandwidth product. Even if the input buffer had no
> offset voltage, its response time still depends on overdrive.
>=20
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>=20
>=20
t'ta
rajat
____________________________________________ =20
RAJAT CHAUHAN =20
STMicroelectronics Pvt. ltd. =20
(Intelligence behind Intelligence)
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