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Re: What are these capacitors for?
On Thursday, October 26, 2017 at 10:38:22 PM UTC-7, rickman wrote:
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Yes - within the BGA package itself.  That's not on the silicon but within the PCB and bond wires to the die.  This was a Virtex4 FPGA without in-package decoupling. Supposedly the package resonance was typically in the 30-50MHz for this type of packaging.

Since it is within the package it can only be influenced slightly by what is done on the PCB itself.

The modern FPGAs such as Virtex Ultrascale that I am now using have in-package decoupling and only require a small handful of capacitors on the PCB.

kevin

Re: What are these capacitors for?
On Fri, 27 Oct 2017 07:14:01 -0700 (PDT), kevin93

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Some capacitors are truly on-chip, tens to hundreds of nanofarads.


--  

John Larkin   Highland Technology, Inc   trk

jlarkin att highlandtechnology dott com
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Re: What are these capacitors for?
On Friday, October 27, 2017 at 9:36:41 AM UTC-7, John Larkin wrote:
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ackage decoupling and only require a small handful of capacitors on the PCB
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I assume it is the on-chip capacitors resonating with the interposer/bond w
ire inductance that was causing the basic problem. I was surprised how low  
the resonance frequency was.  We couldn't measure it (apart from failures i
n operation of the FPGA). Once we found a fix we didn't research it any mor
e.  We didn't have a problem with a follow on design using Virtex5.

kevin

Re: What are these capacitors for?
On Fri, 27 Oct 2017 11:52:27 -0700 (PDT), kevin93

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We normally bring a few FPGA i/o pins out to test points or SMA
connector footprints. One could then program a pin to ground or to the
bank Vcc and snoop the noise there. Some chips would probably let you
access Vcc_core that way too. Since there are many Vcc_core pins, one
of them could be (carefully) sacrificed as a snoop, too.




--  

John Larkin   Highland Technology, Inc   trk

jlarkin att highlandtechnology dott com
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Re: What are these capacitors for?
On Friday, October 27, 2017 at 5:40:59 PM UTC-7, John Larkin wrote:
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We didn't try that, thats a good idea.

Even then it would probably only allow the voltage going into the interposer to be snooped and not the on-die voltage.  Maybe Xilinx has some way of bringing that out.

This was about 10 years ago on an obsolete version of the product so things have changed a lot by now.

kevin

Re: What are these capacitors for?
kevin93 wrote on 10/27/2017 10:54 PM:
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I'm not sure why you think that.  The I/O pin is not really connected to the  
power rails in the chip package.  They have some parasitic capacitance and  
inductance, but other than that LRC filter you should see the voltage from  
the chip power distribution.

--  

Rick C

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Re: What are these capacitors for?
On Friday, October 27, 2017 at 11:25:57 PM UTC-7, rickman wrote:
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Sorry - I was referring to the core voltage that was causing the problems.  

You are right about the I/O pin being able to sense the I/O bank voltage on die but I don't see how there is a way to sense the core supply.

kevin

Re: What are these capacitors for?
kevin93 wrote on 10/28/2017 12:36 PM:
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Yes, you are right.  I was forgetting the I/O pins are powered from separate  
circuits in most FPGAs.

--  

Rick C

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Re: What are these capacitors for?
kevin93 wrote on 10/27/2017 10:14 AM:
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If I understand correctly the fix was to modify the FPGA design to reduce  
the large fluctuations in power changes at the resonant frequency?  Was this  
a particularly large Virtex4?

--  

Rick C

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Re: What are these capacitors for?
On Friday, October 27, 2017 at 3:19:38 PM UTC-7, rickman wrote:
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his  
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Yes - that was the main fix.  I also added a bunch of decoupling and tuned  
the supply voltage that gave incremental improvements.  Once we had it work
ing i didn't go back and remove those other changes so I'll never know if t
he code change would have been enough by itself.

I don't remember which particular Virtex 4 device it was but it was one of  
the largest in the family and it was fairly full - we needed 5 FPGAs togeth
er to house all the logic we needed.  Also I think what made it particularl
y bad was that a large proportion of the logic was in the same pipeline wit
h the 4 out of 5 clock activity rhythm. I suspect if there was more random  
logic in the device it wouldn't have been so much of an issue.

kevin

Re: What are these capacitors for?
kevin93 wrote on 10/27/2017 8:22 PM:
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It's just interesting that it even happened.  How did Xilinx figure it out?  
Was this something they were aware *could* happen?  One of the reasons the  
decoupling recommendations are so over the top is because of the wide  
variety of consumer designs.  Likewise, it would have been inevitable that  
sooner or later a customer would have a design that stimulated this resonance.

--  

Rick C

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Re: What are these capacitors for?
On Friday, October 27, 2017 at 5:30:18 PM UTC-7, rickman wrote:
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They didn't tell how they knew just gave us various bits of information and suggestions on how to fix the problem - it took us a couple of PCB spins to solve on a board that had must have had more than $10,000 of parts on it.

The $25,000 FPGAs I'm using now are even pricier and they are not top of the range!  Better make sure I connect that power the correct way round :-)

kevin

Re: What are these capacitors for?
On Fri, 27 Oct 2017 20:00:15 -0700 (PDT), kevin93

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You can reuse FPGAs, if they're expensive enough to warrant the pain
and clearly yours are.  That doesn't mean you don't have to check and
recheck your footprints, though.  ;-)



Re: What are these capacitors for?
On 10/24/2017 08:43 PM, John Larkin wrote:
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OP gave the impression that they were all physically located together on  
the PCB as well, which wouldn't make a lot of sense in this application.

Re: What are these capacitors for?
On Tue, 24 Oct 2017 21:26:18 -0400, bitrex

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They are grouped together on the schematic, but I did not find any
cluster of them on the PCB. There are a lot of caps on the board, and
I'm finding an occasional 0.1 here and there. Since the board does not
have them marked as (C38) or whatever, I dont know which ones they are.

If I had to do any actual work on the circuits, I'd need a much more
detailed schematic. But I found what I needed on this poorly made
drawing to determine the resistance of the slider pot I have to replace.


Re: What are these capacitors for?
On Tue, 24 Oct 2017 17:43:18 -0700, John Larkin

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This makes the most sense......

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Re: What are these capacitors for?
John Larkin wrote:
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That's basic common knowledge, but when I asked about mobo replacement  
caps, people said it's ok to use long leads if the cap doesn't fit.

It reminded me of the AC family with center power pins because the  
corner pins had too much inductance.  Something about this isn't clear.




Re: What are these capacitors for?
On Tue, 24 Oct 2017 22:10:59 -0400, "Tom Del Rosso"

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Things like this always puzzle me. At one time it seemed that a circuit
consisted of an active component (transistor, chip or tube), with
several passive components connected to them (caps, resistors etc) to
form an amplifier or oscillator, or whatever. Needing mutliple caps like
this seem rather odd. Like, who decides where they need to be?

Over the years, I worked on more old tube equipment than I have the
newer stuff. Things were never so critical in tube circuits. Some high
frequency RF circuits needed under chassis shielding and tube shields on
the top, but for the most part, wires fell where they laid and
everything worked. I always knew to keep the AC power cord away from
components and other wires as much as possible, but that was about all
it took.  

Even the early transistor qquipment did not seem as critical. Now we
have what sometimes seem to be too many things to consider. Just making
a PCB seems to be a major accomplishment, because you cant just make a
board so all the parts fit together as needed, but the traces have to be
kept away from other traces and so on.... I often wonder who and how
long it takes to design a PCB to fit all these parts.  

This Equalizer is around 30 years old, so I am sure it's worse now to
design things. One good thing, Peavey still supports and has parts for
this equalizer, that gives me a good opinion of them. We already have a
glut of old electronics polluting the earth and we sure dont need to add
more, especially when the stuff is still useful and works fine, with
only a little care. This one works quite well, sounds great, but had a
bad power switch. Somehow one of the slider pots was broken off too, and
it's the GAIN control for one of the channels, so it's important.

This thing has 26 chips. I believe they are all Op amps. There are 6
transistors, and a few diodes. The rest is mostly all caps, and
resistors which form the filters for the freq bands. Beyond that, it's
just the power supply parts. I like the fact that all of the chips have
sockets. But I sure wish they had labeled the parts on the board.


Re: What are these capacitors for?
On 10/25/2017 01:23 AM, snipped-for-privacy@tubes.com wrote:
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The go from the supply pins to ground as close to the IC package as  
possible for the reasons Phil stated. It's not rocket surgery. Forrest  
Mims also recommends it in his beginner books on op amp circuits:

<http://www.rogtronics.net/files/ebooks/Engineer 's%20Mini-Notebook%20-%20Op%20Amp%20Ic%20Circuits.pdf>

(see bottom left, page 6.)

Re: What are these capacitors for?
On Wed, 25 Oct 2017 00:23:42 -0500, snipped-for-privacy@tubes.com wrote:

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Low frequency stuff is not critical; just worry about ground loops and
hum pickup, like in the tube days. It helps now, not having a filament
supply.

But some parts are fast nowadays, like logic that switches in
picoseconds or GHz opamps. There, layout and trace impedances and the
speed of light really matter.

A square foot of dense logic, with switching power supplies and a
couple FPGAs and some fast analog stuff and thermal worries, say a
thousand parts, might be a 10-layer board and could take six weeks or
so to lay out manually. That becomes a team project.

I like reference designators silkscreened on my boards, but they take
as much area as the parts.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


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