VCC width

[aleksa]

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.

 

[Tim Wescott]

aleksa wrote:

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
www.wescottdesign.com

 

[George Herold]

On May 17, 3:35 am, "aleksa" <aleks...@gmail.com> wrote:

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 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.)

I make two-layer digital boards only.
That makes it harder. The 'pros' will tell you, four layers with one

inner the ground plane and the other inner for power.

If I now adjust my VCC tracks to 50mils,
is there any chance of some problems?

Problems, there always seems to be problems. Wider traces may or may
not cure your problems.

And should I adjust? All prior boards work...

If it ain't broke don't fix it.

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.

Good luck and have fun....
George H.

 

[aleksa]

"George Herold" <ggherold@gmail.com> wrote in message news:fa2a60df-7a8a-45b0-943a-d5da4e5b4594@s41g2000vba.googlegroups.com...
On May 17, 3:35 am, "aleksa" <aleks...@gmail.com> wrote:

If I now adjust my VCC tracks to 50mils,
is there any chance of some problems?

Problems, there always seems to be problems. Wider traces may or may
not cure your problems.

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?

 

[aleksa]

"Tim Wescott" <tim@seemywebsite.now> wrote in message news:UNmdnem-7tvm7WzWnZ2dnUVZ_tIAAAAA@web-ster.com...

aleksa wrote:
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.

Perhaps you can hang yourself with it?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

 

[Bob Myers]

On 5/17/2010 2:14 PM, aleksa wrote:

"George Herold"<ggherold@gmail.com> wrote in message news:fa2a60df-7a8a-45b0-943a-d5da4e5b4594@s41g2000vba.googlegroups.com...
On May 17, 3:35 am, "aleksa"<aleks...@gmail.com> wrote:

If I now adjust my VCC tracks to 50mils,
is there any chance of some problems?


Problems, there always seems to be problems. Wider traces may or may
not cure your problems.

I didn't say I *have* problems, I asked
if widening a VCC *may make* problems.

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.

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.

And at that point, SOMEONE should've thought long and
hard about just why that should be.

Bob M.

 

[aleksa]

Thanks

 

[Tim Wescott]

aleksa wrote:

"Tim Wescott" <tim@seemywebsite.now> wrote in message news:UNmdnem-7tvm7WzWnZ2dnUVZ_tIAAAAA@web-ster.com...
aleksa wrote:
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.

Perhaps you can hang yourself with it?

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
www.wescottdesign.com

 

[John Larkin]

On Mon, 17 May 2010 15:36:36 -0700, Tim Wescott <tim@seemywebsite.now>
wrote:

aleksa wrote:
"Tim Wescott" <tim@seemywebsite.now> wrote in message news:UNmdnem-7tvm7WzWnZ2dnUVZ_tIAAAAA@web-ster.com...
aleksa wrote:
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.

Perhaps you can hang yourself with it?

Perhaps I could comment that there _are_ stupid questions?

There are no stupid questions, only stupid people.

John

 

[John Larkin]

On Mon, 17 May 2010 09:35:50 +0200, "aleksa" <aleksazr@gmail.com>
wrote:

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.

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

 

[George Herold]

On May 17, 4:14 pm, "aleksa" <aleks...@gmail.com> wrote:

"George Herold" <ggher...@gmail.com> wrote in messagenews:fa2a60df-7a8a-45b0-943a-d5da4e5b4594@s41g2000vba.googlegroups.com...

On May 17, 3:35 am, "aleksa" <aleks...@gmail.com> wrote:

If I now adjust my VCC tracks to 50mils,
is there any chance of some problems?
Problems, there always seems to be problems.  Wider traces may or may
not cure your problems.

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?"

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.

 

[krw@att.bizzzzzzzzzzzz]

On Mon, 17 May 2010 20:43:41 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Mon, 17 May 2010 09:35:50 +0200, "aleksa" <aleksazr@gmail.com
wrote:

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.


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.

Altera's Max-2 CPLDs are available as 3.3V only, though I still wouldn't
attempt a double sided board design with them.

Make the ground and Vcc as wide as you can, and stitch them with lots
of bypass caps. Lots of luck.

John

 

[Jasen Betts]

On 2010-05-17, aleksa <aleksazr@gmail.com> wrote:

"George Herold" <ggherold@gmail.com> wrote in message news:fa2a60df-7a8a-45b0-943a-d5da4e5b4594@s41g2000vba.googlegroups.com...
On May 17, 3:35 am, "aleksa" <aleks...@gmail.com> wrote:
If I now adjust my VCC tracks to 50mils,
is there any chance of some problems?

Problems, there always seems to be problems. Wider traces may or may
not cure your problems.

Anyway, I'll rephrase the question:
can widening a VCC line make problems?

In a working, but marginal, system any change can "make problems".

http://catb.org/jargon/html/magic-story.html







--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

[Bob Masta]

On Mon, 17 May 2010 22:14:47 +0200, "aleksa"
<aleksazr@gmail.com> wrote:

"George Herold" <ggherold@gmail.com> wrote in message news:fa2a60df-7a8a-45b0-943a-d5da4e5b4594@s41g2000vba.googlegroups.com...
On May 17, 3:35 am, "aleksa" <aleks...@gmail.com> wrote:
If I now adjust my VCC tracks to 50mils,
is there any chance of some problems?

Problems, there always seems to be problems. Wider traces may or may
not cure your problems.

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...

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.

Anyway, I'll rephrase the question:
can widening a VCC line make problems?

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
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
DaqMusic - FREE MUSIC, Forever!
(Some assembly required)
Science (and fun!) with your sound card!

 

[aleksa]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message news:b734v55bqgpfdrp97cb011jauogvcqelj9@4ax.com...

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.
Make the ground and Vcc as wide as you can, and stitch them with lots
of bypass caps. Lots of luck.

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.

 

[aleksa]

"Jasen Betts" <jasen@xnet.co.nz> wrote in message news:hstkha$3g4$4@reversiblemaps.ath.cx...

In a working, but marginal, system any change can "make problems".
http://catb.org/jargon/html/magic-story.html

Considering how complicated things can be,
I wonder sometimes how anything works.

 

[aleksa]

"George Herold" <ggherold@gmail.com> wrote in message news:7857193f-af04-4393-96e6-

Tim Wescott probably knows a lot more about electronics than I do....
If you're nice he might help you.

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".

 

[aleksa]

"Bob Masta" <N0Spam@daqarta.com> wrote in message news:4bf27919.1501573@news.eternal-september.org...

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.

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.

 

[George Herold]

On May 18, 8:58 am, "aleksa" <aleks...@gmail.com> wrote:

"George Herold" <ggher...@gmail.com> wrote in message news:7857193f-af04-4393-96e6-
Tim Wescott probably knows a lot more about electronics than I do....
If you're nice he might help you.

I can't be nice to someone who isn't nice to me.

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.

I asked "can widening a VCC cause problems" and not
"how to calculate needed trace thickness".

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.

 

[krw@att.bizzzzzzzzzzzz]

On Tue, 18 May 2010 15:08:34 +0200, "aleksa" <aleksazr@gmail.com> wrote:

"Bob Masta" <N0Spam@daqarta.com> wrote in message news:4bf27919.1501573@news.eternal-september.org...
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.

L is usually more important than R.

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.

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.

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.

As wide as possible, absolutely. Cuts down on the L.