[SI-LIST] Re: Common Clock and Source Synchronous Timing Margins
- From: Itzhak Hirshtal <hirshtal@xxxxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Sun, 27 Jan 2002 11:52:27 +0200
Hello jothsna and Riazi,
I've assembled together all of your different responses because they have
relatioships to each other.
To jothsna -
The answer to your question depends on the chip vendor's definition! I'm
accustomed to a definition which is in respect to the strobe <<output>> pin, but
Mr. Riazi's definition for a source-synchronous design seems to be in respect to
the strobe <<input>> pin, that's why he has both Tco_data and Tco_strobe (see
his answers to me). I also understand that he refers to a design in which the
strobe signal is used to capture the data on the <<opposite edge>> than the edge
on which the data is transmitted. <<I>> refered to a source-synchronous bus
which samples the data at the receiver on the same dege as the one used to
transmit the data.
Regards
--------------------------
jothsna R wrote:
>
>
> Hi Itzhak,
> What does Tco physically mean. Is it the delay of data signal within the chip
> with respect to the strobe signal as seen at the pin.
>
> thanks,
> student.
To Abe Riazi -
My response above to jothsna is relevant also to you!
Abe Riazi wrote:
> Hi Itzhak:
>
> Please find inserted my replies to each of your comments.
> I have added defintion for "Tdelay" which you asked about as part of your
> comment #2.
> Itzhak Hirshtal Wrote
> >
> > Here are some comments, which I hope will be valuable:
> >
> > (1) It seems to me that the source-synchronous method requires the data
> bits to be
> > transmitted a little AFTER the strobe signal, otherwise you'll probably
> find it difficult
> > to avoid setup AND hold violations. Don't you agree?
> >
>
> No.
>
> Every source synchronous design I have expreienced have either
> transmitted the data and strobe simulataneously or strobe being after the
> data. The latter is especially desirable when the goal is to center the
> edge of
> strobe at middle of the data signal for optimium performance.
See my response to jothsna above.
>
>
> Additionally, in order to avoid timing degradation across variations of
> temperature, voltage and process; it is essentail for PCB routing
> topologies of data and strobe to be identical (and on the same board
> layers).
> >
> > (2) Also, it seems your equations for the source-synchronous method
> contain some errors and
> > inappropriate parameters:
> > - Tco_strobe is not a usable parameter; what is it referenced to? As I
> understand it,
> > only Tco_data is relevant, and it is referenced to the strobe signal
> itself in the data
> > sheet.
> > - What is Tdelay exactly? Can you explain? It seems a non-relevant
> parameter as well.
> > - One must use Tco_min in the hold equations (for source-synchronous
> as well as common
> > clock). I shall call this parameter Toh_data.
> > - In my opinion, the 2 equations should be:
> >
> > Tsetup_margin = Tcycle + Tflt_strobe - Tco_data - Tsetup -
> Tflt_data
> >
> > Thold_margin = Tflt_data - Tflt_strobe + Toh_data - Thold
> >
> Your setup and hold equations have omitted Tco_strobe, and Tdelay
> paramteres; however, several documents
> (including book by Stephen H. Hall, et al.) indicate these timing paramters
> need to be preserved in the source synchronous timing fromulas.
>
> Tco (time from clok to ouput) represents time required for a bit (data or
> strobe) to appear at ouput of a latch (or buffer) after it has been clocked
> in.
> Tco_strobe, and Tco_data usually have different values.
See my response to jothsna above.
Tdelay equals delay between data and strobe clocking and,
Tvb = Tco_data - (Tco_strobe + Tdelay)
Where, Tvb (called valid before) is time interval that data signal is valid
before strobe.
> >
> >
> > (3) You haven't mentioned the opposite direction equations, which are
> different for the
> > source-synchronous case (Tflt_strobe sign is inverted).
> >
> True
>
> I referred to one direction in order to keep the discussion simple.
> However, in reality the bus agents can be bi-directional and we should
> remember:
>
> i. Tco_strobe and Tco_data are driver parameters.
> ii. Tsetup and Thold belong to receivers.
>
> >
> > (4) To be very accurate one must use the minimum and maximum parameters
> and simulation
> > results as appropriate.
> >
> Correct!
>
> Again, for purpose of simplicity, I had assumed a nominal corner and
> constant values for Tco_strobe, Tco_data, Tflt_strobe, and Tflt_data.
> However, each of these paramters can have different Minimum, Typical
> and Maximum values.
>
> Subsequently, for worst case analyses, the (Tflt_strobe - Tflt_data )
> expression in setup equation becomes (Tflt_strobe_min - Tflt_data_max ).
> Similarly, (Tflt_data - Tft_strobe) in Hold equation gets replaced by
> (Tflt_data_min - Tft_strobe_max).
>
> The nominal Tco_strobe and Tco_data also need to be properly replaced
> by their minimum or maximum values.
> >
> >
> > (5) I would like to comment on the solution you propose for setup and hold
> failures. It is
> > very difficult to compensate for them by lengthening or shorting the data
> traces, since
> > they are generally numerous. It is much easier to do the same for one of
> the clock signals,
> > thus generating a deliberate skew. This, of-course, assumes the problem is
> one-way, and the
> > solution does not cause a new problem.
> >
> >
> When there are a group of data signals sharing the same strobe and only one
> or two of the data signals fail setup margin, then it is preferable to
> shorten the data signal(s). However, when many or all of the signals fail
> setup it
> is desirable to lenghen the strobe signal.
>
> Similar reasoning can be applied to Hold margin failures to ascertain which
> is simpler - lenghtening data or shortening the strobe.
>
> I agree with your comment that for a bi-directional bus, when fixing timing
> problems of one direction we need to ensure that violations will not occur
> when signalling in opposite direction.
> >
> > (5) The Interconnectix (ICX) simulator also provides a means for
> automatically compensating
> > for the load difference between the test load and the actual PCB trace
> load. But this
> > method depends, of course, on the manufacturer to supply the appropriate
> test load
> > parameters. Since the IBIS model, on which ICX simulations are based,
> assumes a certain
> > configuration of test capacitance and resistance, this presents a problem
> where the
> > manufacturer uses a different configuration. For example, I encountered a
> manufacturer who
> > claimed he used a current-source test load, with a different current load
> for the high and
> > low states of the tested outputs. I had to use approximations in order to
> be able to
> > simulate these outputs using an IBIS model. Has anyone encountered such a
> problem and have
> > a solution?
> >
> I had mentioned XNS becasue of familiarity with the software. It is possible
> that other prgrams such as ICX can also automatically compenstate for the
> buffer delays. The important point is that we need to understand our tool
> and know whether this delay subtraction is to be achieved automatically or
> manually.
>
> The test load parameters (Cref, Rref, Vref, Vmeas) have been previously
> discussed in this forum, for example "Looking Inside IBIS Model",
> dated May 15, 1999.
>
> Best Regards,
>
> Abe Riazi
> ServerWorks
--
Itzhak Hirshtal
Elta Electronics
POB 330 Ashdod
Israel 77102
Tel : 972-8-8572841
Mobile: 972-64-238631
Fax : 972-8-8572978
email: hirshtal@xxxxxxxxxxxxx
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