When I started designing PCBs, I was given an advice to route GND with 50mils and VCC with 25mils.
But now I read on the net that VCC should be wide, also.
I make two-layer digital boards only.
If I now adjust my VCC tracks to 50mils, is there any chance of some problems?
And should I adjust? All prior boards work...
Maybe I should note that all my previous boards were built around discreet chips, GALs, SRAM and Z80, but am now moving towards 180 MHz ARM and FPGA.
I need a rope to hold up a sack containing all my belongings. How thick should it be?
What? You want to know how many belongings I have, and how much they weigh, and whether any of them are alive and may make the sack bounce and jump around? Why oh why would you want to know that, just to tell me how big a cruddy rope needs to be?
OK: I'll make a deal with you:
_You_ tell _us_ enough about what you're doing (expected current draw, more about the parts that you're going to use, whether you're running any analog parts, etc.) Then maybe _we_ will have enough information to tell _you_ the answer that you need.
And I'll go figure out that damn rope size myself.
-- Tim Wescott Control system and signal processing consulting
I don't do much digital stuff, but high speed is high speed. You want a ground plane. And then make the power traces as wide as possible. The idea is to keep trace inductance as small as possible. (Which also implies short traces.)
That makes it harder. The 'pros' will tell you, four layers with one inner the ground plane and the other inner for power.
Problems, there always seems to be problems. Wider traces may or may not cure your problems.
If it ain't broke don't fix it.
Good luck and have fun.... George H.
I didn't say I *have* problems, I asked if widening a VCC *may make* problems.
The guy who suggested I should use 25mil VCC had an old board, with DIPs, with thick GND and thick VCC lines (some 100mils), with caps on all chips, but the board didn't work correctly. After he had removed the thick VCC line from one of the chips, and soldered a thin wire, the board worked correctly.
He also said something like "If VCC lines are not thick, device will draw spike current from caps, rather than the VCC line". Something like that. I'm no expert, and I also have to translate it to English...
Anyway, I'll rephrase the question: can widening a VCC line make problems?
should it be?
and whether any of them are alive and may make the
tell me how big a cruddy rope needs to be?
about the parts that you're going to use, whether
information to tell _you_ the answer that you need.
Perhaps you can hang yourself with it?
Simple answer: no, not unless there are other things wrong with the design. Having both Vcc and ground on full planes is the norm for 4-layer-and-up boards, you know.
And at that point, SOMEONE should've thought long and hard about just why that should be.
Bob M.
Thanks
should it be?
and whether any of them are alive and may make the
tell me how big a cruddy rope needs to be?
about the parts that you're going to use, whether
information to tell _you_ the answer that you need.
Perhaps I could comment that there _are_ stupid questions? Stupid questions of the first sort are ones that you already know the answer to. Stupid questions of the second sort are the ones that are poorly asked.
I'm sorry I tried to help -- I'll remember your level of gratitude if I'm tempted to help you in the future.
-- Tim Wescott Control system and signal processing consulting
snipped-for-privacy@s41g2000vba.googlegroups.com...
"> Anyway, I'll rephrase the question:
No that makes no sense at all.... some weird time delay thing maybe. If you do something to your circuit and it responds in a way you don't understand, then spend some time to understand it, else it will come and 'bite' you in the future.
Above 100 MHz it's harder... what's the wavelength at 100MHz? (in your circuit board) If you are 1/10 of that distance 'strange' things can start to happen.
Tim Wescott probably knows a lot more about electronics than I do.... If you're nice he might help you.
George H.
should it be?
and whether any of them are alive and may make the
to tell me how big a cruddy rope needs to be?
about the parts that you're going to use, whether
information to tell _you_ the answer that you need.
There are no stupid questions, only stupid people.
John
It's going to be interesting to try to get an ARM or an FPGA to work on a 2-layer board. The Vcc and ground current spikes can be horrific for a fast, many-pin chip, and a wimpy routed (non-plane) ground structure will cause nasty ground bounce noise.
Four layers is about the minimum for decent power and signal integrity on a high-speed board (signals, ground, power pours, signals.) 6 or 8 if you have high density or BGA parts. Most FPGAs need three power supplies, heavily bypassed, and that will be really hard to do on a
2-layer board.Make the ground and Vcc as wide as you can, and stitch them with lots of bypass caps. Lots of luck.
John
Altera's Max-2 CPLDs are available as 3.3V only, though I still wouldn't attempt a double sided board design with them.
In a working, but marginal, system any change can "make problems".
--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---
Your translation is just fine, and what he said makes a certain amount of sense. He's saying that the resistance of the Vcc lines plus the bypass capacitors act as RC filters on each chip, so if one chip draws a current spike it is "handled" from its own local C with less disturbance of the Vcc for other chips.
The question is whether this approach is better than using a fat Vcc trace (low R) whose voltage doesn't move around as much in response to spikes in the first place. My guess is that his approach may have made a difference on a marginal design, but the next design might have a completely different behavior.
Maybe, if you have a marginal design or layout. But if you are looking for a general design rule, I'd go with fat ground and Vcc as the best bet to avoid the most problems.
Best regards,
Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis
I have a working FPGA board, Spartan II XC2S50 TQFP144. If I replace the PC with ARM, I could even use VQFP100. The freq is not much, 40 MHz, and I doubt I'll go higher than that.
I've placed vias on every VCC/GND pair and placed caps on the bottom side. The bottom side is almost free of signals, so there's no problem connecting the power.
As for the ARM @ 180 MHz... well let's wait and see. I hear other people made it.
Considering how complicated things can be, I wonder sometimes how anything works.
I can't be nice to someone who isn't nice to me.
I asked "can widening a VCC cause problems" and not "how to calculate needed trace thickness".
Its easy when you have a 4-layer board, but when you have a 2-layer board you can't always use thick GND *and* thick VCC. That was another reason to use thiner lines.
Since I now use smd parts only, all signals are on top layer so there's plenty of space on the bottom.
Thats why I asked if I should widen my VCC lines from now on.
Oh, that's too bad. You need to have a thick skin around here sometimes. Wait till you say something stupid and Phil A. jumps on you. Still there is a vast store of collective knowledge that you can tap into.
Yeah, your original question was not all that clear. Though I think we get it now. It sounds like you haven't run into many "ground" problems yet. So perhaps it's too soon to learn about massive 'power pours' as John L. calls them. My guess is that as you move to higher frequencies you'll find that for glitch free operation you will need not just wide ground traces but a real ground plane.
George H.
L is usually more important than R.
You can't always use a 2-layer board. I wouldn't *think* of it for any high-speed digital devices, like FPGAs (though the I/Os can often be tuned down). Even four layers is cutting it close.
As wide as possible, absolutely. Cuts down on the L.
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