[MOVED] EAGLE schematics says connected; PCB layout says otherwise

[randomshinichi]

Hey everybody, I've been laying out my EAGLE schematic, and everything looks OK. But when I try to layout the PCB, many bypass capacitors show up not connected to the pin they're supposed to bypass, but are instead connected to other things which the voltage supply rail is connected to.

For instance, CB3 here, which is obviously connected to the net connected to the DVDD pin of a MAX11040, shows up not connected to the MAX11040 at all in the ratsnest. Instead, it shows up as connected to the MAX5318, which is true, but only through the 3V3 voltage supply rail, as the schematic in the attached pdf will show you.

Why is this happening? It's obviously connected to the pin, how do I know that EAGLE is thinking what I'm thinking?

 

[keith1200rs]

Re: EAGLE schematics says connected; PCB layout says otherwise

The ratsnest will always show the shortest route. It is up to you to put the capacitors next to the appropriate chips. As far as the software is concerned all 3V3 connections are identical, as are all DGNDs etc.

Keith.

 

[randomshinichi]

So is the ratsnest really useful then? The whole point of having a bypass capacitor is to have it right next to the pin and connected via as short a trace as possible, right? What's the use of putting the bypass capacitor next to the pin in question if the ratsnest tells me they're not going to be directly connected?

 

[keith1200rs]

Don't confuse placement with connectivity. The placement of components is up to you. The connections can be made either automatically or manually but physically placing components near to one another in the schematic never makes any difference to connectivity in the netlist on any of the different layout packages I have ever used.

The ratsnest is very useful but you do need to refer to the schematic as you lay it out.

Keith

 

[randomshinichi]

So, what you're telling me is to place all bypass caps as close as possible to their respective pins, tell EAGLE to recalculate the ratsnest and hope to *** that a direct airwire appears between the pin and its corresponding bypass cap. And if it doesn't, rest assured knowing that somehow, electrically, it's the best possible solution anyway.

 

[keith1200rs]

Not really. Yes, place the capacitors next to the appropriate IC and recalculate the ratsnest. It will then show the shortest route. However, when you start to route it Eagle will continually recalculate the shortest path as you move the cursor. It is a guide, but it is not saying that is the route you must take. Often I find the shortest route might be to another component or a via but that doesn't mean it is the way I will go - I start routing from the capacitor to the IC pins.

The only way to get a feel for it is to try it and practice.

Keith