A Question on Schematic Style

Just a quick question here…

I’m placing the components for my TinyFPGA based stepper motor controller board (see the prior articles here firsthere next, and here last). Specifically, I’m finding places for all of the capacitors. That’s where the question of schematic style comes in. 

When I first started using CAD software, sometime shortly before the stegosaurus roamed the land, I would position component symbols on the schematic near the chips they belonged with. Doing so made it easy, come layout time, to remember which capacitors go where.

Since then, though — and I don’t remember when I started this — I’ve started following the practice of grouping capacitors on the schematic, as I’ve done on this sheet to the right. All of the capacitors are up at the top, shown connected to V+ and ground rails.

That’s not a problem when they’re all 0.1uf bypass capacitors and each chip just takes one. However, with higher speed and more complex chips, multiple bypass caps or different values are often required on each chip.

Now, when I go to layout, I need to go find my component data sheets again (they really should never be very far away) and re-figure out what combinations of bypass capacitors go to which pins on what chips.

I like the cleaner schematic that results from grouping bypass caps, but it’s adds pain and a bit of opportunity for error during layout.

What style do you use and why? Am I an idiot for doing it this way? Wait. Don’t answer that last question.

Duane Benson
Six of one and 12 × 5 × 10-1 of another

How Should You Mark Your Diodes?

Current flows through a diode from the anode to the cathode – it will pass current only when the potential on the anode is greater than the potential on the cathode. This is mostly true, but not always.

For the common barrier diode, or rectifier, it’s a pretty safe bet. However, with a zener diode, or  TVS, it’s not true. And, that is why marking a diode, on your PC board, with the plus sign (+) is not good practice.

Take a look at the schematic clip below.

 

1 designers nb figure 1

 

 

 

 

 

Once you put this circuit on to a PC board, you could legitimately place a plus sign on the anodes of D3 and D4, and another on their cathodes. In the next schematic clip, you could legitimately place both a plus sign, and a minus sign on the anode of D9.

1 designers nb figure 2

We don’t know what you had in mind, and, we don’t have the schematic. If you use the practice of marking diodes with a (+) on the anode, we don’t have any more information than if you didn’t mark it at all. The same holds for using a minus (-) sign. It really doesn’t give us any information.

So how should you mark your diodes? The best method is to put the diode symbol next to the footprint. on the PC board, as shown below. You can also use “K” to indicate the Cathode, of “A”, to indicate the Anode. “K” is used because “C” could be mistaken for “capacitor.”

D5, in the illustration on the right, would be the preferred methodFigure 1. D7 will work as well. If you don’t have enough room on the board due to spacing constraints, you can put the same information in an assembly drawing.

Ambiguity is the enemy of manufacturers everywhere. Read a bit more on the subject here, or here.

Duane Benson
Help stamp out and eliminate redundancy, and maybe ambiguity, or maybe not