[SI-LIST] Re: DDRAM BUS Testing

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: "Ihsan Erdin" <erdinih@xxxxxxxxx>
  • Date: Fri, 04 Aug 2006 05:01:36 -0700

Ihsan, in most systems we have many capacitors.  In many of the more 
expensive systems, we also tend to have relatively thin power 
cavities tying capacitors and loads together.  So, removing one cap 
for the sake of measurement yields a tolerable error that we can 
pretty much calibrate out once we consider the capacitor and IC 
distribution, and the composition of the plane cavity(s).  As a 
matter of fact, if you are a disciple of Istvan's DET methodology, 
the impedance across the planes can be very uniform, at least in 
theory.  In the ideal incarnation of that methodology, current 
disturbances go off to disturbance heaven at the cavity edge where 
the resistance of the DET network absorbs the noise, preventing it 
from reflecting back into the cavity.
Now if your Charles who has to build boards where 4 layers is a 
luxury, the situation can be very different.  There, the interconnect 
characteristic impedance has a great deal of variation depending on 
distance from any given bypass cap.  Those applications present 
sometimes vexing PDS design and measurement issues.  If there is any 
rock to stand on, it is very shaky.

What I would like to emphasize is that looking at the planes even 
where you can, tells a potentially very different story than what we 
care about first, which is the die.  Looking at the planes is 
valuable when trying to:

1. See if the power system is in real trouble.  Whatever noise is on 
the planes there is more at the die.
2. Figure out how much cross pollution we get between ICs,
3. Assessing EMC issues, or
4. Assessing that always lively topic: referencing high speed signals 
to disparate reference planes.

But if you want the full story you really want to know what the die 
sees.  There is no good substitute.

We know from models and measurement that disturbing power at the die 
in approximate order of noise magnitude

1. Alters timing,
2. Causes erroneous logic levels.
3. Causes metastable and / or false switching events
4. Damages the IC

The jitter issues that you face have a myriad of contributing causes, 
one of which is noise coupled onto the PLL supply voltage rail.  Al 
Neves and Ransom Stephens can and do keep people busy for days 
explaining: the causes, how to find them, and how to correct / mitigate them.

Regards,


Steve.
At 04:33 AM 8/4/2006, Ihsan Erdin wrote:
>Steve,
>
>I agree with your comments about the 5% tolerance. Despite 
>occasional references in some research papers, it's a long-time 
>outdated and useless figure of merit for contemporary designs. If my 
>design couldn't meet it I'd throw it in the garbage without further tests.
>  My observations with high speed designs are that the biggest 
> impact of the noise is on the jitter and PLL operation. Especially 
> for OC48 and higher telecom designs, the metric is much more 
> complicated than +/- 5% tolerance on the VRM output. I think the 
> research in this area should be directed to establish such a spec 
> rather than beating this subject to death with copycat papers.
>I have a comment about your proposed measurement of noise coming 
>from an IC. By removing the cap you change all the system dynamics. 
>How useful could such a measurement be to represent the noise at 
>that point? In other words, why should I care for how much noise I'd 
>see at that point without the cap?
>
>Regards.
>
>Ihsan
>
>On 8/3/06, steve weir <<mailto:weirsi@xxxxxxxxxx>weirsi@xxxxxxxxxx> wrote:
>Charles, 5% noise tolerance spec at the planes or caps will someday
>go the route of the infamous 50pF TTL load spec.  It tells us only a
>little bit about what we really want to know, and that is what is
>happening at the die.  The value of looking at a capacitor site or
>even a dedicated probe into the planes depends on what you want to know.
>
>First, even if the cap is very close to the chip, that location still
>tells you about power distribution near that locale.  Even if the cap
>is right next to the IC connected through thin planes, or on the
>backside of the PCB, once we take the Z axis wild ride up into the
>chip package, the results can be very different.  For example we can
>have dead quiet PCB planes and but dangerously noisy internal rails
>at the die.  When Lee Ritchey complains about effects attributed to
>packages, that is the sort of symptom to watch out for:  quiet PCB
>but horribly noisy die.  All that interconnect from the die out works
>with the planes and bypass cap network to form a low pass filter than
>disguises what is happening at the die.
>
>If you want to know how much noise from a given IC is propagating out
>the planes, then you can sample the planes.  Removing a capacitor and
>probing that site differentially provides a pretty good picture of
>what the planes see in that locale.  As much as we would like to
>think of the planes as bedrocks of stability, they are far more
>volatile.  In applications like yours where bypass caps have to do a
>higher proportion of the work than in boards with many layers, the
>value of that information is limited.
>
>If you want to know power supply noise across Vdd/Vss at the chip you
>really need to have a diff pair of Vdd/Vss from the die
>available.  With core supplies you have no choice but to burn a
>Vdd/Vss pair.  A pair of outputs is not a bad substitute for looking
>at noise on an I/O rail.
>
>As Mark pointed out, the signals that you bring out are subject to
>cross talk from whatever is near them in the package and the PCB.  It
>doesn't matter what kind of signals they are.
>
>If you want to see what the ambient is for a given receiver, then you
>need to bring out that signal line and its Vref differentially.  That
>can be very difficult as Vref is often short changed pin-wise.
>
>Regards,
>
>Steve.
>At 12:52 PM 8/3/2006, Grasso, Charles wrote:
> >Hi Mike=20
> >
> >One thing that has vexed me in the past ( and continues
> >to vex me actually ) is the measurement of noise at the
> >decoupling cap. If the cap is some distance from the
> >chip - the the noise seen at the cap will not faithfully
> >represent the noise at the chip.=20
> >
> >I'd like to ask you (and the folks on the reflector)
> >your opinion on the "best" location for measuring and qualifying
> >power noise.
> >
> >Thanks!!
> >=20
> >
> >Best Regards
> >Charles Grasso
> >Compliance Engineer
> >Echostar Communications Corp.
> >Tel: 303-706-5467
> >Fax: 303-799-6222
> >Cell: 303-204-2974
> >Pager/Short Message: <mailto:3032042974@xxxxxxxxx>3032042974@xxxxxxxxx
> >Email: <mailto:charles.grasso@xxxxxxxxxxxx>charles.grasso@xxxxxxxxxxxx;
> >Email Alternate: <mailto:chasgrasso@xxxxxxxx>chasgrasso@xxxxxxxx
> >
> >
> >-----Original Message-----
> >From: 
> <mailto:si-list-bounce@xxxxxxxxxxxxx>si-list-bounce@xxxxxxxxxxxxx 
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >On Behalf Of Michael Rose
> >Sent: Wednesday, August 02, 2006 5:07 PM
> >To: <mailto:Edi.Fraiman@xxxxxxxxxx>Edi.Fraiman@xxxxxxxxxx; Vinu 
> Arumugham; <mailto:si-list@xxxxxxxxxxxxx>si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: DDRAM BUS Testing
> >
> >Edi,
> >
> >I assume you're worried about signal integrity (not =3D
> >logic/functionality). Some troubleshooting tips to try:
> >
> >1. measure noise on the Vref lines for both the FPGA and memory. I don't
> >=3D
> >remember offhand but I belieive max is around 30mV. Micron Tech has a =
> >=3D
> >good DDR layout guide.
> >
> >2. carefully measure 1.8V or 2.5V Vcc and Vtt noise (use an AC-coupled =
> >=3D
> >50 ohm coax soldered directly to a local bypass cap site.
> >
> >3. Tie a probe to the DDR clock at the memory to trigger a high-speed =
> >=3D
> >scope (> 4GHz). It's nice to have software write a test loop to generate
> >=3D
> >various read and write bit patterns on the memory bus (especially =3D
> >alternating 00s and FFs). Probe each memory line and note under/over =3D
> >shoot, jitter, Tsu and Th. Remember you have to probe D/DQ at the memory
> >=3D
> >for write data and at the FPGA for read data.
> >
> >4. For Xilinx FPGAs, pay particular attention to undershoot (see their =
> >=3D
> >appnote where they suggest running the memory bus drivers at .3V below =
> >=3D
> >Vcc to prevent reverse biasing the clamps).
> >
> >I bet you'll find either excessive power supply noise, Vref noise, Vtt =
> >=3D
> >noise, or under/over shoot
> >
> >Good Luck, Mike
> >
> >
> >-----Original Message-----
> >From: <mailto:si-list-bounce@xxxxxxxxxxxxx>si-list-bounce@xxxxxxxxxxxxx
> >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Fraiman, Edi
> >Sent: Wednesday, August 02, 2006 4:01 PM
> >To: Vinu Arumugham; <mailto:si-list@xxxxxxxxxxxxx>si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: DDRAM BUS Testing
> >
> >
> >Yes, FPGA support JTAG.
> >But JTAG frequency is low. JTAG test wasn't working because we are out
> >of spec (minimum DDR operating frequency).
> >
> >=3D3D20
> >Best regards,
> >=3D3D20
> >Edi Fraiman
> >
> >
> > > -----Original Message-----
> > > From: Vinu Arumugham [mailto:vinu@xxxxxxxxx ]=3D3D20
> > > Sent: Wednesday, August 02, 2006 3:54 PM
> > > To: Fraiman, Edi
> > > Subject: Re: [SI-LIST] DDRAM BUS Testing
> > >=3D3D20
> > >=3D3D20
> > > The FPGA does not support JTAG?
> > >=3D3D20
> > > Thanks,
> > > Vinu
> > >=3D3D20
> > > Fraiman, Edi wrote:
> > >=3D3D20
> > > >Hi,
> > > >=3D3D20
> > > >
> > > >I'm working on a design that includes FPGA with DDRAM controller =3D
> >and=3D3D20
> > > >several DDRAM chips. The traces goings direct from FPGA to=3D3D20
> > > DDRAM with=3D3D20
> > > >necessary pulll up resistor to Vref. (SSTL interface)=3D3D20
> > > without any debug=3D3D20
> > > >connector.
> > > >
> > > >=3D3D20
> > > >
> > > >Sometimes we have productions problems. It is very difficult to =3D
> >find=3D3D20
> > > >what bit in bus between FPGA and DDRAM  is shorted or disconnected.
> > > >
> > > >Could somebody give any tips how it's possible debug DDRAM busses =3D
> >in=3D3D20
> > > >terms of production issues.
> > > >
> > > >=3D3D20
> > > >=3D3D20
> > > >Best regards,
> > > >=3D3D20
> > > >Edi Fraiman
> > > >=3D3D20
> > > >
> > > >
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