[gta02-core] Next phase: the schematics

[Werner Almesberger]

I think we've progressed well enough with the symbols and their review
that we can attack the next phase now. That would be the schematics.

There may still be a few details to be sorted out in the symbols, but
I think they won't cause much of an upset. One exception may be the
ATR0635, see below.

Since we don't have a lot of space on our A4 sheets, I think we need
to break down things into even smaller pieces than I did in the
expanded view. A sensible breakdown may look roughly like this:

- CPU:
  - CPU device control: crystal, reset, board revision inputs, debug
    board.
  - CPU I/O: LEDs and AUX button, all the miscellaneaous signals that
    go elsewhere.
- CPU-POWER: power inputs and caps
- PMU:
  - PMU core: regulator in/out, caps, I2C, crystal, POWER button
  - PMU peripherals: battery, VB switch, STBY_1V2 regulator
- MEMORY: let's hope it all fits on a single sheet ...
- AUDIO:
  - AUDIO internal: codec, power supply
  - AUDIO external: speakers, mike, jack, 
- USB: like on the expanded view
- IO: might need picking apart. SDIO should to go the sheet with
  the SD/SIM holder.
- LCM: hopefully fits on a single sheet
- SD/SIM: like on the expanded view, plus the CPU's SDIO block
  coming from the "IO" sheet
- GPS: I suspect that we'll have to split the ATR0635 into power
  and the rest. The RF side gets simpler, so it should fit with
  the ATR0635. So that would be:
  - GPS power: SI1040X lower half of the ATR0635, bypassing, chokes.
  - GPS main: all the rest
- BT: ought to fit
- WLAN: ought to fit as well

I liked how we did the symbols, namely that someone would go ahead and
start drawing, and then we'd discuss the issues that came up. I think
both PMU and memory would be good sheets to start with, because they
will be the same as in Openmoko's design.

My own schematics style is fairly space-grabbing: to each "pin", I add
four grid units of "wire" before changing direction. Related "wires"
are spaced by two grid units and unrelated "wires" by four units. The
space between joins and pins or between joins is also four grid units.
Perhaps there's a more compact style that looks good or even better.

One thing I'd caution against are cross-shaped joints. They'we very
confusing, particularly since KiCad sometimes only makes very small
dots to mark a joint. So it's better to have only T-shaped joints.

Meanwhile, I'd like to focus on writing that scripted footprint editor
I've started a while ago. That should bridge the gap we currently have
between interactively drawing footprints (which is a very frustrating
task) and generating footprints with a general-purpose scripting
language (which yields nice results, but gets a bit cryptic).

So it would be good if someone could go ahead and give the
schematics a first try. Once we've settled on a common style, we can
then also go through the design changes ... and maybe even document
them ;-)

- Werner