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Topic: PCB design without having to do schematic first? (Read 5460 times) previous topic - next topic

ralphd

I'd like to design a board using a QFP32 AVR.  I've tried Eagle and KiCad, and can't figure out a way to just design the board.  I can do it by creating a schematic with the parts I want, then a netlist, then PCB.  What I want is a way to just place place and route footprints.  Is there a way to do this in KiCad or Eagle, or do I need to use some other software?

CrossRoads

www.ExpressPCB.com will let you do that.
You may have to create the footprint first, I don't recall what their surface mount library had for footprints.
Very easy to use.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

ralphd

I'd like to send my final design to OSHPark, Hackvana or Itead Studio, so I'd need gerber output.  It looks like ExpressPCB only saves to their proprietary format.

graynomad

Quote
It looks like ExpressPCB only saves to their proprietary format.

Yes I think that's the case.

Doing a PCB with no schematic is not recommended IMO, but if it's a simple design it should be OK. Altium won't let you as it needs to know what is connected even to run manual traces, you would have to assign net names to everything appropriately.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

westfw

You can do it in EAGLE.  Just start up the board editor and place "packages" rather than devices, connect pads with the "signal" wire (or not, I guess) and lay down tracks.

It's a really bad idea, IMO.   There is tremendous value to the redundant error checking you get by having the tools check both the schematic AND the board, AND making sure they agree.

ralphd


You can do it in EAGLE.  Just start up the board editor and place "packages" rather than devices, connect pads with the "signal" wire (or not, I guess) and lay down tracks.

It's a really bad idea, IMO.   There is tremendous value to the redundant error checking you get by having the tools check both the schematic AND the board, AND making sure they agree.


Thanks.  I'll try that in Eagle.

What kind of error checking could I get for a qfn-32 breakout board?
http://www.adafruit.com/products/1163
I want to make one with 0.5" spacing and ICSP header, and maybe a pad for a 1uF cap.

CrossRoads

What does the ICSP header connect to? Just pads? Or to certain pins?
Once you connect to pins, you're better off with a schematic.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

ralphd


What does the ICSP header connect to? Just pads? Or to certain pins?
Once you connect to pins, you're better off with a schematic.


I think I see where you're going.  The ICSP header will connect to pins (MOSI/MISO/SCK...), and in a schematic I could label them, and the CAD sofware would verify that they match up.  With only 6 pins to verify, manually verifying the pinout seems like a lot less work than doing a separate schematic.

The best would be a single drawing with schematic and layout information.  I should be able to choose a QFN ATtiny88, and drop it on my PCB.  Then I should be able to choose a 6-pin ICSP header and drop it on my board.  Then if I try to draw a trace from VCC on the ICSP to GND on the ATtiny88 pads, the CAD should warn me.  Maybe one day...

graynomad

I can see that with such a simple design winging it without the schem would be OK, that said for the 2 minutes it takes to do one I would.

Why the 50mil spacing? Just curious.

Quote
maybe a pad for a 1uF cap.

And a 100nF for decoupling, maybe 2-3 depending on the chip.

_______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

ralphd


Why the 50mil spacing? Just curious.

More space on the breadboard. I like the Pro Minis, but the 0.6" spacing leaves only two rows empty on a typical breadboard.  0.5" would be tough to do with a QFP, but with the QFN it's no problem.  And 0.5" still leaves enough room to plug it in on top of a 0.3" DIP.

Quote
maybe a pad for a 1uF cap.

And a 100nF for decoupling, maybe 2-3 depending on the chip.
[/quote]
I thought that was just a rule of thumb.  I'm a rather obsessive minimalist, so I rarely do things "just in case". Ceramic chip capacitors are very low ESR, so I would think think a single 1uF would be fine for smoothing any voltage fluctuations on an 8Mhz part.
I'd love to see some tests showing power/ground on a 100Mhz scope with different decoupling caps to see how much of a difference it makes.

TheCoolest

DipTrace also allows you to create/route a PCB without a schematic. And it has tons of landing patterns, so you don't have to create your own components for the most part.

ralphd


DipTrace also allows you to create/route a PCB without a schematic. And it has tons of landing patterns, so you don't have to create your own components for the most part.

Thanks.  I'll check it out.

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