[OLPC-devel] making DC measurements with "mic in", progress

[Jaya Kumar]

On 6/17/06, Miller Puckette <mpuckett at imusic1.ucsd.edu> wrote:
> Hi OLPC developers (particularly Mark perhaps)
>
> Simon and I have got as far as to verify that, with VREFD asserted on the
> AD1888 codec, whatever voltage we put on the MIC input actually gets to
> pin 21 of the AD1888 chip.  I think this implies among other things that
> we are indeed using the "modded" board (in fact Jim found the solder blob).
>  We only made two measurements, 0V and 2V, but that seemed to make the case
> that we've made it to the codec.

Cool.

>
> Unfortunately, we still can't seem to get DC results out of the CODEC; some-
> body further downstream is high-pass filtering the signal.  I doubt ALSA

This might be a silly question. I interpret what you said about the
step function earlier to mean you are driving _down_ from 2V to 0V and
you are seeing a dying exponential. On the scope, when probing pin 21
as you drive your step signal, do you see the step waveform itself?
Just to double check, when you measure resistance through the short
you've made, it's ~0 right. I'm just wondering if the cap that you've
shorted with the blob is discharging resulting in the dying
exponential. Even so, the schematic says it's 1uF so it'd be hard to
get an RC of 0.5s. If you stick a 1Hz sine wave in, does that come out
looking okay?

> would do such a thing and am only hoping there's another setting we need
> to change in the codec.  I'm going to start looking through the codec doc
> to see if I can find anything, but if anyone else can think of something,
> I'd be eager to hear it.


I'm wondering if the ADC antialiasing filter caps with the 270pF on
pin 29 and 30 on page 13 have any effect.

Other than that, I can't think of anything concrete. I assume you're
setting the sampling rate from ALSA and not writing those regs
directly. Maybe try varying the rate to see if it has any effect if
you haven't already.

jaya