[gta02-core] Audio ECNs, a bit of progress

[Werner Almesberger]

Some news from the ECN front:

- I finished ECN0031, which describes how to use the codec interrupt
  for HOLD and JACK_INSERT instead of involving the CPU directly.
  This is from Joerg's "Mega-ECN".

- I updated ECN0015 (removal of the Calypso's rear entrance through
  the audio jack) with the changes that have actually been done.

  I wonder if we shouldn't get rid of R4405/7 completely instead of
  making them 0R. We'll have to add some EMI and ESD protection, so
  there will be room for interrupting the circuit, if necessary.

  Once the final fate of R4405/7 is decided, we can move ECN0015
  to "Done".

- we have one regression: the ESD protectors D3001 and D3002 (at
  the front speaker) were lost in translation.

I still have more items to process from the "mega-ECN". Here's my
"to do" list:

- new ECN: add ESD to jack

- buzz prevention: remove 22 pF caps around jack, place EMI filters
  (C4403, C4404, C4405), remove common-mode choke B4102

- bass fix: add component, remove R4405/7 (0R for us)

- bass fix: big caps (~100 uF), tantalum, no cer

- new suppression:
  we might consider making each of C3012/13 a 4u7 + 47p couple
  as that should rather completely short any RF induced to the "hot" side of
  those C (chip side) to GND

- buzz fix: provide place for bigC

- rear ECN: merge 18 pF + 47 pF into 56 pF

- new ECN: swap rear choke / caps

- new ECN: remove C3020

- new ECN: increase input caps from 100 nF to 1 uF (ceramic)
  (1uF) according to wolfson appnotes
  - mic out caps
  - RXN/RXP (modem to codec)
  - MONO1/MONO2 (codec to modem)

- new ECN: route LOUT1/ROUT2 back to LINE1/LINE2

- make sure C3012/13 are well separated from micbias and any other source of RF
  iirc there's even a special note about those 2 C in 8753 app notes

Lots of small critters ...

- Werner