[gta02-core] Values and units in schematics

Werner Almesberger werner at openmoko.org
Tue Jun 30 06:42:24 CEST 2009

The question on how exactly to write values in schematics came up again.
We still have to settle on a common notation.

Here are some possible variations:

- of decimal points (1.2 k) vs. the multiplier (1k2)
- if the multiplier is used as a decimal point, how to handle numbers
  that don't need a multiplier
- whether to capitalize multipliers (2 K) or not (2 k)
- whether to include units (4.7 uF) or not (4.7 u)
- use of a space between number and unit

In SI-style, one would write 4.7 uF [1].

Using the multiplier as decimal point is quite common and IMHO
increases the readability. E.g., 4k7 isn't easily confused with 47 k. 

A common convention is to use the unit instead of the multiplier if
none is needed, e.g., 0R or 4R7.

Capitalizing the multiplier is tricky. It can cause confusion in some
extreme cases, e.g., 10 mOhm vs. 10 MOhm, but it also improves
general readability, e.g., 10 nF and 10 uF vs. 10 NF and 10 UF.

The Openmoko schematics tend to omit the unit for resistors and
capacitors, are inconsistent for inductors, and tend to put it in all
other cases.
Openmoko schematics also have constructs like 6N8H, which are
consistent, but seem a little hard to parse.
The Openmoko schematics don't use a space between number and unit.

I think we should continue following the multiplier-as-decimal-point
convention, because it's just too easy to overlook a decimal point.
Also bringing in the unit as a decimal point seems reasonable. (Only
relevant for R, V, and maybe A.)

I'd lean slightly towards always capitalizing multipliers, also
because the font size in schematics tends to be chosen such that
upper case is readable, but with less regard to lower case.

I don't think we have any milliohms anywhere, so we can avoid that
problem :)

I'm not sure whether to include units in general (as a reminder) or
to omit them. Perhaps it's best to omit them for simple components
(resistor, cap, inductor, etc.), and to include them for anything
more complex, e.g., varistors.

I'm undecided about the space. I tend to use a space in text (mails,
comments, etc.), but when it gets more low-level (identifiers or
component values), I don't use a space. A nice example for why a
space is generally a good idea would be "10Ohm". Of course, this can
be defused by writing "10R".

[1] http://en.wikipedia.org/wiki/International_System_of_Units#SI_writing_style

- Werner

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