Author Topic: Understanding RATSNEST with regards to Ground plane  (Read 1551 times)

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Offline mindentropy

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Understanding RATSNEST with regards to Ground plane
« on: April 02, 2021, 10:19:16 am »
Hi All,

I am a newbie in PCB design and I am analysing TI Launchpad MSP-EXP430G2 board. I have some confusion regarding ground plane in Eagle. In the following image I have clicked "RATSNEST" to see how the ground plane is connected to the GND pads. In the following image I understand that the red fill is the Ground plane.



Now when I click the left mouse again at the edge polygon, the image changes to the following:



I know that the Top is dark red and the Bottom is blue. In both the images I am confused as to what are the blue patches? Is it a bottom ground plane? Does it mean I have 2 Gnd planes?

Also why are the top traces and the GND plane have the same color when the GND plane lies one layer below the Top layer?
 

Online Kean

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Re: Understanding RATSNEST with regards to Ground plane
« Reply #1 on: April 02, 2021, 10:33:53 am »
This is a 2 layer PCB.  You have ground planes on the top and bottom layers (1 &16) - shown in red and blue - that fill any unused copper per the design rules, and calculated when you click on rats nest.

Any blue you see is the bottom layer showing through gaps in the top layer.  You can use the layer settings to turn on/off layers, or change the current active layer to bring it to the top.

If you configure your board for 4 layers in design rules (not available in free version), then you will have access to two internal layers that can be used for routing or power/gound planes.  They will show as another 2 colours, by default yellow-ish and orange.
 
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Online Kean

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Re: Understanding RATSNEST with regards to Ground plane
« Reply #2 on: April 02, 2021, 10:36:48 am »
What tool did you have active when you clicked to get the second image?
 

Offline mindentropy

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Re: Understanding RATSNEST with regards to Ground plane
« Reply #3 on: April 02, 2021, 06:33:27 pm »
Quote
You have ground planes on the top and bottom layers (1 &16) - shown in red and blue - that fill any unused copper per the design rules, and calculated when you click on rats nest.

I was only aware of having GND plane in a separate layer and thought each plane has its own layer. Can we have GND and Vcc planes in the same layer?

Quote
What tool did you have active when you clicked to get the second image?

I clicked on the "Show" tool and left clicked on the GND Polygon twice.
 

Online Kean

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Re: Understanding RATSNEST with regards to Ground plane
« Reply #4 on: April 03, 2021, 12:12:28 pm »
A plane is just a polygon that (typically) covers most, or all, of the PCB area.  Generally when people talk about a ground or power plane they mean a dedicated layer, so your example images would more correctly be called ground (or Vcc) fill.  This is commonly done on 2 layer PCBs to get some of the benefits of a dedicated plane without the extra cost of a multi-layer PCB.

If you have a polygon creating ground or power fill on the same layer as other traces, then it will be broken up by the traces - as show in your images.  This can mean that you get islands (Eagle calls them orphans) that are not connected to the rest of the net.  A setting on the polygon determines if Eagle will hide or show those orphans.  Having orphans on can be misleading, but DRC should warn you about it.  Planes with their own dedicated layer will only be broken up by vias, holes, slots, etc - but on a dense board that plane can look like swiss cheese.  Via stitching is often used between layers to reduce impedance and connect any islands.

I was only aware of having GND plane in a separate layer and thought each plane has its own layer. Can we have GND and Vcc planes in the same layer?

You can have multiple polygons on the same layer connected to different nets (e.g. GND and Vcc), but obviously they cannot overlap.  Look at more sample boards that have been published to get a feeling for how these are used - generally for power distribution or thermal control.

Note that in Eagle you should use different "rank" settings on polygons on the same layer to make sure they don't overlap.  Rank 1 is the most important, rank 2 less important, etc.  If you forget this and the polygons create a short then it will be highlighted by DRC.
 

Online Kean

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Re: Understanding RATSNEST with regards to Ground plane
« Reply #5 on: April 03, 2021, 12:20:47 pm »
Quote
What tool did you have active when you clicked to get the second image?

I clicked on the "Show" tool and left clicked on the GND Polygon twice.

I know I've seen something like that before, but I can't reproduce it right now.  It must be a difference in preferences.  It is confusing!
I get the ground fill polygon highlighten in cyan instead of red, and the red traces remain.  For the bottom layer it is yellow instead of blue.
 


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