How should chips with pins on bottom be drawn?

Question

When you are drawing a schematic for an IC, and there are pins in the bottom center (in my case I am drawing for the BGM13S32F512GA-V2R), how should they be drawn?

I have looked into some beginner tutorials, but everything I've found uses ICs with pins along the side only. I am using OrCAD Capture.

I have also heard a debate between whether you draw the schematic based on the datasheet, or you draw it based on how the pins are used (GND on bottom, VCC on top, everything else on sides... or however).

Answer 1

I'll start by answering your second question. Generally you should place pins in your schematic symbol so that it produces the neatest schematic possible. Usually this means power on the top, ground on the bottom, inputs on the left, and outputs on the right.

I generally place the pin for the bottom pad logically based on its function. 9 times out of 10 the pad will be for ground, so I generally place it at the bottom of my schematic component. If it's used for the supply, I put it at the top. Simply put, just place it in the most logical location that will make your schematic cleanest and easiest to follow (usually this means minimizing wire lengths, and following the "signal flow direction").

Answer 2

A schematic is not meant to be a drawing of a circuit, and shouldn't be used that way. The "pins" in a schematic just show connections. Put them wherever they need to go to make the drawing understandable, and don't worry about what they convey physically.

Answer 3

The style used for creating documentation depends upon the goal of the drawing. When I create a "Schematic", I try to make the drawing useful for Troubleshooting and understanding the function; Visual likeness to reality is intentionally not part of my thinking. When drawing a "Wiring diagram", more thought goes into making the physical reality clear, especially for Enclosures, wire and cable routes. For board level drawings, I use sort of a hybrid approach where the physical layout is a small factor in that the IC's are drawn as a box and kept as 1 unit in the drawing.

The approach I would use for your question would depend again on the type of drawing I was creating. If it's a location diagram, then draw the IC as it appears in real life. If it's a schematic, the simple approach would be to draw it as a box with no regard for Physical accuracy. Then draw lines terminating at, or slightly within, the edge of the box. Add numbering either just inside or just outside the box. The lines need not be in the same order as they are on the IC. Draw them whatever way is most useful.

Answer 4

The data sheet you referenced essentially answers the question, if you look at figures 3.1, 3.2, 3.3, and 7.2.

Organize your complete circuit diagram in a similar way, though (obviously) you don't need to draw the internal architecture of the chip.

This is analogous to the simpler case of drawing a transistor or FET. You use a conventional symbol, without bothering about the physical layout of the pins of any particular device. Different transistors are packaged with every possible permutation of three pins around a circle or in a straight line!

Answer 5

It may be tempting to put pins in the same order as they are on the package. After all, it makes it easier to compare the schematic and the PCB.

But there is a significant danger of mistakes. Especially voltage supply pins are usually spread on all sides of the package, and may not follow any clear order. Then when you are connecting them in schematic, mixing up or forgetting some is much easier than if all VSS pins are next to each other, and all VDD pins are next to each other.

I was chasing a short-circuit under BGA chip for quite some time. Then I realized it was a schematic error with a completely different chip, which had its symbol drawn in the order the pins are on the package: