Ask MAKE: Voltage annotations

Kevin asks:

What do all those little subscript letters and numbers after V on circuit diagrams mean Vcc, Vee, Vss mean?

I have to admit, I didn’t really know the full answer to this one, so I looked it up. I found a page on the solarbotics website explaining the whole shebang: Vcc and Vdd mean that that point in the circuit is directly connected the power source, and Vee and Vss mean that point it is connected to ground. It went on to say:

Apparently this terminology originated in some way from the terminals of each type of transistor, and their common connections in logic circuits (i.e., Vcc is often applied to BJT collectors, Vee to BJT emitters, Vdd to FET drains, and Vss to FET sources). This notation then carries across to integrated circuits — TTL ICs were originally based on BJT technology, and so often use the Vcc / Vee terminology; CMOS ICs are based on FET technology, and so often use the Vdd / Vss terminology.

The absolute distinctions between these common supply terms has since been blurred by the interchangeable application of TTL and CMOS logic families. Most CMOS (74HC / AC, etc.) IC data sheets now use Vcc and Gnd to designate the positive and negative supply pins.

Image is a snippet of the schematic for SparkFun’s Blue SMiRF.

Have you got additional information? Post it in the comments! Have a question for Ask MAKE? Shoot me an email at becky@makezine.com or drop us a tweet! We’d love to answer your questions on anything MAKE-y.

5 Responses to Ask MAKE: Voltage annotations

Andy Peters on April 30th, 2009 at 10:17 pm said:

It goes back to the days of discrete transistors.

Vcc is connected to a bipolar transistor’s collector.
Vee is connected to a BJT’s emitter.
Vdd is connected to a FET’s drain.
Vss is connected to a FET’s source.

It’s all kinda vestigial now, and more modern nomenclature usually indicates the actual voltages.

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mightyohm on May 1st, 2009 at 1:00 am said:

The subscript notation is used a lot in electrical engineering.

Vce = collector – emitter voltage of a bipolar transistor
Vbe = base – emitter voltage
Vbc = base – collector voltage
and so on.

Amusingly, Vcc is not collector – collector voltage (which would be zero) but the voltage of the collector with respect to ground. I’m not sure where the extra c came from, maybe it’s supposed to indicate more than one collector. Who knows!

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clvrmnky on May 1st, 2009 at 8:29 am said:

Is there a good reference for reading digital device datasheets?

I keep forgetting what (for example) the CS’ nomenclature means (a CS with a line over it) which is sort of key when reading the timing diagrams and logic tables.

I think it means that CS (“Clock Strobe” or whatever) is kept high relative to ground until you want to have it do something, in which case you bring it low. At least, this is how I have reasoned it out, but doing things backwards: reading the timing diagrams and logic table and figuring out when the pin is to be low or high. Not ideal.

But a reference that talks about reading datasheets like this is something I can’t seem to find in any of the CMOS refs I have. It seems to fall into the cracks between technical books and howto manuals.

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- vivi on May 1st, 2009 at 6:43 pm said:
  
  CS usually means “chip select” and is used to enable a communication bus. The bus is active only if this signal is active.
  
  A bar over a signal name means that this signal is active low. So, if your signal is “CS”, you need to send a logic “1″ to the pin to enable it. If it is “CS bar” you need to send a logic “0″.
  
  Reset signals are also commonly active low.
  
  I think I’ve read this information the first time in the old National Semiconductor databooks. Unfortunately they’re not made any more and very difficult if not impossible to find.
  
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vivi on May 1st, 2009 at 6:39 pm said:

CS usually means “chip select” and is used to enable a communication bus. The bus is active only if this signal is active.

A bar over a signal name means that this signal is active low. So, if your signal is “CS”, you need to send a logic “1″ to the pin to enable it. If it is “CS bar” you need to send a logic “0″.

Reset signals are also commonly active low.

I think I’ve read this information the first time in the old National Semiconductor databooks. Unfortunately they’re not made any more and very difficult if not impossible to find.

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