next topic »

[on:]

I've been having this problem so I whipped up a very simple test case to help test it out and explain it and hopefully someone can tell me what I am doing wrong here.  This is in DipTrace but I am guessing whatever I am doing wrong is stupidity and is universal over all EDA tools.  Basically,  DipTrace is simplying the design when going from Schematic to PCB for my decoupling capacitors.  I can see why it does it but I don't know how to tell it that my specific wires in the Schematic need to remain on the PCB because these parts need to remain local to the pins I have them assigned to.  Anyway, three pictures is worth 3,000 words so here goes:







You can see that DipTrace has basically made these caps parallel and the caps have lost their despiking capacity because they are no longer directly connected to the VCC pins they are supposed to despike.  My first thought was to change the net name of the GND wires going into the caps, but changing it for one changes it for all.  Plus, I don't know if that is the real solution anyway.

Any hints?

[Reply #1 on:]

The caps will be in parallel electrically  no matter what. What you need to do is move them to be physically close to the pins where you want the decoupling to occur.

[Reply #2 on:]

Thanks for the advice.  When I rearrange it the autorouter makes saner choices.  I think the major problem is that I am relying on the autorouter and not having very good placement before doing that.  Probably I should not be autorouting at all but my skill level is somewhat constrained right now.  This is the job it did this time:

[Reply #3 on:]

Good placement to start is the key, whether autorouting or not.
Even when I hand route, I try the autorouter first. If it's really struggling, that's a pretty good sign that your placement is not good, or that your placement _might_ be okay and you just have a ton of parts to deal with.

You also need to figure out ground planes, that will eliminate a lot of the traces for ground pins and will help a Lot!
Can do things like add vias named Gnd also to connect the top & bottom layers, will help to bring Gnd to pins that might otherwise be isolated by signals around them.
Using smaller vias will help also - like 12mil vias for 10mil traces.

[Reply #4 on:]

I have zero faith in auto-routing.  I guess it's ok when signal integrity, board use, and placement are all non-issues.  Otherwise, I hand-route because I care.  :-)  Placing each individual trace gives me the chance to really think about what the signal is doing, what it's near, and what interactions I may or may not experience based on choices I make.

Kinda like changing your car's oil.  You can outsource that, but when you do it yourself, there's a bond between man and machine.  (Often that bond can be dissolved with some citrus cleaner, though.)