Board layout for FPGA
in [FPGA]
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From: Symon on 5 Feb 2010 14:25 On 2/5/2010 7:15 PM, Mike Harrison wrote: > worked just fine..... didn't do any noise measurements though.... > > A few years ago I saw a very amusing talk at London Dorkbot - in an attempt to bring old board games > up to date, James Larson created "Motherboard Operation" - players take it in turns to snip > components from a working, running PC motherboard until it stops working... > That's like Muntzing! Named after Madman Muntz. :-)
From: glen herrmannsfeldt on 5 Feb 2010 16:42 Mike Harrison <mike(a)whitewing.co.uk> wrote: (snip) > up to date, James Larson created "Motherboard Operation" - players > take it in turns to snip components from a working, running PC > motherboard until it stops working... > This was accompanied by "PC PSU Buckaroo" - players choose and add > more and more loads to an old PC power supply until it fails.... I haven't tried it recently, but it used to be that PC power supplies would fail at zero load. I did it once (I don't remember why) and smoked one. (Yes, real smoke.) The original PC/AT had an optional hard disk drive. If you didn't buy one there was a load resistor on the power supply to meet the minimum load requirement. You would think that the AT motherboard would take enough current, but it seems not. -- glen
From: glen herrmannsfeldt on 5 Feb 2010 16:53 rickman <gnuarm(a)gmail.com> wrote: > On Feb 5, 8:45 am, Symon <symon_bre...(a)hotmail.com> wrote: (really big snip) >> with a thick centre core and routed powers. This way the internal signal >> layers are shielded. I tend to agree. The ssggss stack I suggested >> because I almost always use laser drilled micro-vias on my boards, so I >> need two signal layers on the outside. Also, my enclosures do the EMC >> shielding. With standard vias, sgssgs is probably better. (snip) > As to the return current having to "jump" between layers being a > problem, if you use the ssgpss stackup and have the power and ground > very close rather than widely spaced, the capacitive coupling allows > the signal to switch between them without issue. In fact, when > splitting a plane for multiple power sections, the return current will > switch from one power plane, to the ground plane and back to the next > power plane as if they were all one plane. This is because of the > capacitive coupling between layers. Of course this only works for the > highest frequency components of the signals, but that's all we really > care about, no? > -------+ +-------> Return Current > =======| |======== Power Planes > | | > +--+ > =================== Ground Plane Yes. Actually, I believe that for the really highest frequency components, that they are supplied by the plane itself. (Especially as signals won't be switching at exactly the same time.) The slightly lower ones, but only slightly, will take the path you mention. Frequencies with wavelength shorter than the long path around won't be able to take that path. The lower frequencies are still there, though, but at low enough levels that ordinary bypass capacitors and interplane capacitance will take care of them. -- glen
From: Symon on 5 Feb 2010 20:42 On 2/5/2010 9:53 PM, glen herrmannsfeldt wrote: > > Yes. Actually, I believe that for the really highest frequency > components, that they are supplied by the plane itself. (Especially > as signals won't be switching at exactly the same time.) > Hi Glen, If these highest frequencies are supplied by the planes, why do Xilinx and Altera put capacitors on the BGA substrate? That must cost money. Are they wrong? Thanks, Symon.
From: glen herrmannsfeldt on 5 Feb 2010 21:24
Symon <symon_brewer(a)hotmail.com> wrote: > On 2/5/2010 9:53 PM, glen herrmannsfeldt wrote: >> Yes. Actually, I believe that for the really highest frequency >> components, that they are supplied by the plane itself. (Especially >> as signals won't be switching at exactly the same time.) > If these highest frequencies are supplied by the planes, why do Xilinx > and Altera put capacitors on the BGA substrate? That must cost money. > Are they wrong? OK, not quite the highest frequencies, but almost. The highest that get through the inductance of the package and past the internal capacitors. I think it isn't so hard to calculate the impedance of an infinite plane with a hole (via) as a function of frequency. That should partly answer the question. -- glen |