MICBIAS buzz injection

[Werner Almesberger]

Joerg and I discussed today how to verify that the MICBIAS buzz rework
was done properly. Joerg had a very interesting idea: by toggling the
codec's MICBIAS output, we should be able to create noise that would
vary depending on whether the rework has been done. He asked me to
adapt one of my I2C-tweaking programs for this purpose.

Here's a little program that hammers the Wolfson as fast as it can,
making MICBIAS oscillate at about 50Hz:
http://svn.openmoko.org/developers/werner/wolfhammer/

The result looks like this, on a non-reworked A6, measured on the
codec side of R4305:
http://people.openmoko.org/werner/a6-nrw.png

The amplitude seems pretty small (and I have plenty of ambiental
noise), so I'm not sure if this is good enough. To verify that
MICBIAS was indeed being toggled, I added a 1k resistor between the
measurement point to ground (i.e., parallel to R4305 and C4302),
which yielded this:
http://people.openmoko.org/werner/a6-nrw-1k.png

Joerg, I hope this is useful.

- Werner