A question about Xu?'s PSU components

Werner Almesberger werner at openmoko.org
Mon Dec 13 15:00:45 EST 2010


Adam Wang wrote:
> Thus the time people reviewed them is unnecessary from concepts of tool.
> A7108E5R-12 is better than A7108, isn't it? Because they are totally not the
> same: one is 1.2V fixed output, the other is that we don't know unless I
> jump into schematic to see.

Hmm, the workflow should be "read schematics, then look up things you
need with dsv". So I wouldn't worry about the BOOKSHELF to schematics
direction.

Also note that dsv and boom don't know of each other so far. Their
names do not have to be the same. (See below.) In the future, I'd like
to have a boom -> data sheet connection, which may or may not involve
dsv, but that's still a while out.


However, there is a much more basic problem in the schematics, and
that's that some chips have an incomplete name. E.g., the A7108 in
U12 should be at least A7108-12, etc.

One question here is which elements in the name are really needed.
The "full" names, e.g., A7108E5R-12, also have things like tape and
reel and "green" package, which don't really matter in the schematics.

AiT themselves seem to acknowledge this in their data sheet in the
figure named "Schematics 1", where they call the chip "A7108-ADJ" in
the caption. (And they set a bad example by just calling it "A7108" in
the circuit symbol.)

Choosing the "right" name can sometimes be tricky. E.g., some chips
come with different T&R options and such, which you don't care about
at the schematics level. If you over-specify, you may have to undo
all this in boom (e.g., by defining A7108E5R-12 as an equivalence of
A7108E5VR-12, if your supplier only has the "green" package, not the
"non-green").


So, for naming, I would suggest the following:

- dsv: pick a name that concisely describes what's in the data sheet.
  E.g., I think "A7108" is a good name for the A7108* data sheet.

  If a data sheet describes a small number of different parts, you can
  use aliases to name them.

  dsv is meant as an interactive tool for humans, so the name should
  be something that's easy to remember and convenient to type.

  Also, names in dsv can be project-specific. There's no need for them
  to be globally unique. The names or what they refer to can also
  change over time.

- schematics: the name of a component should include enough information
  that one can pick a functionally compatible part without having to
  consult other resources.

  E.g., A7108 is insufficient, because you don't know the voltage or
  whether it's the -ADJ variant.

  If the part comes in different packages but with otherwise identical
  electrical properties, I would not consider it important to include
  the package. (We have the - normally invisible - footprint field for
  that.)

  For some parts, there may be electrical or other properties we depend
  on - e.g., maximum power dissipation - that change with the package.
  In this case, there should be some guidance in the schematics, e.g.,
  a more complete component name, the footprint, or the parameter(s) in
  question.

  In some cases, one may not know the naming scheme well enough to be
  able to tell which name is "sufficient". In this case, I'd just err
  on the side of being overly specific.

- inside boom, you can solve some of those problems with equivalences.
  E.g., you could make AIT A7108E5R-12 an equivalence of AIT A7108E5VR-12

Something to bear in mind is that there are lots of exceptions and errors
that creep into this process as well. E.g., inconsistent vendor part
numbers, distributors changing the format of vendor part numbers, parts
with identical name but different properties, and so on.

This means that we'll never have a "perfect system", but it'll always be
a balance between evils. The goal is to pick the lesser ones :-)

- Werner




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