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While reading a datasheet for an IC I came across the pin voltages being presented as 3V3 or 1V8. What does this representation stand for?

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That's the new politically correct way of writing numbers that would normally have decimal points. Some parts of the world (Germany for example) use a comma to separate the integer and fraction digits. To avoid ambiguity in international situations, some people now put the letter for the units where the decimal point should be. So "3V3" really means 3.3 Volts and "1V8" means 1.8 Volts.

If your audience is English speaking or it is obvious the document or the context is in English, then you are fine using a decimal point normally. After all, using a decimal point is part of the language no less than the words used to describe other things, so this is not ambiguous. In rare cases when numbers are by themselves without a language context, then it's probably best to use the "3V3" type notation. Otherwise, I personally find this notation rather annoying since I have to look at it and think about it rather than the brain parsing it without much conscious thought.

As with most things PC, it's about choosing which group of people to piss off.

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    \$\begingroup\$ You will also see it a lot with resistor values. i.e. 4k7 is 4.7Kohm. \$\endgroup\$ – Kris Bahnsen Mar 14 '12 at 19:35
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    \$\begingroup\$ 3V3 is also shorter than 3.3V \$\endgroup\$ – Pentium100 Mar 14 '12 at 20:00
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    \$\begingroup\$ Ah yes, PC units. I had a friend from SA who when speaking of English vs metric would call them "Christian measurements" vs "heathen measurements". Guess we can't do that any more either ;-) \$\endgroup\$ – JonnyBoats Mar 14 '12 at 20:28
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    \$\begingroup\$ It's worth noting that I've had some problems in SPICE simulations with the "." character in net names (Well, I think it was "." chars, I removed them and it fixed the problem). Using "3v3" or "1v8" is an easy way to clearly describe the voltage while restricting net names to exclusively alphanumeric characters. \$\endgroup\$ – Connor Wolf Mar 14 '12 at 21:37
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    \$\begingroup\$ This notation is neither new (I know if for at least 25 years) nor is it because of the existence of different decimal separation characters. It's for clarity when values are printed on tiny electronic devices like Z-diodes (it's a big difference if its value is 3.3V or a 33V). I think it's just a bad habit that this notation is used also in documents. \$\endgroup\$ – Curd Mar 15 '12 at 10:47
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I've always been told that the reason for using letters or symbols in place of the decimal point is neither for international relations or variable names in code but for clarity in print. The issue being that when datasheets were typed up, or schematics drawn, and then copied/photocopied/faxed/etc the dot may get dropped.

This issue obviously isn't such a big deal these days but using 3V3 or 4R7 is still pretty common. It's really common in older documentation here in the UK (and I'm talking back to kind of era where we really wouldn't have creating documents with legibility to Germans in mind...).

I'm pretty happy transposing commas for full stops in European documents, it's fairly common to find them in European publications (even when they've been translated to English).

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    \$\begingroup\$ I think avoiding that the decimal point might get dropped is the main reason. Especially when values are printed on tiny electronic devices, e.g. Z-diodes. Suppose "3.3V" would be printed on a small 1x2mm cylinder. It could be misstaken for "33V". \$\endgroup\$ – Curd Mar 15 '12 at 10:40
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    \$\begingroup\$ Example: electronicrepairguide.com/images/zener%20diode%20datasheet.jpg \$\endgroup\$ – Curd Mar 15 '12 at 11:28
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It's a naming convention to denote voltages. The period is replaced by the V. So, rather that 3.3V or 1.8V you would write 3V3 and 1V8, respectively. It came about for a couple of reasons:

  • In schematics which involve a device for which code has to be written (microcontroller, FPGA), many code languages use a period or comma as an operator (for example C uses periods to reference structure members). If this is the case, the names of the signals in the code cannot directly follow the schematic net names to which they are connected as it will cause compilation errors. So this scheme was developed to get around this and allow the exact same net to be referenced in the code as in the schematic, allowing for easier readability of the code and a more accurate reflection between the code & schematic.

  • For the reason mentioned above, many schematic editors don't accept periods or commas in the names of the nets because they generate code directly from the schematics or use code to operate directly on the schematics. Both cases can be corrupted by the use of symbols (like periods or commas).

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    \$\begingroup\$ Point 1 sounds like a weak argument since most languages won't let you create symbols starting with a digit. \$\endgroup\$ – Olin Lathrop Mar 14 '12 at 20:33
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    \$\begingroup\$ @OlinLathrop It still could work if your voltage indication is after another name, which is likely (e.g. SetOutput_3V3), where in C code the period would result in a structure type. \$\endgroup\$ – clabacchio Mar 15 '12 at 11:05

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