Michael Zucchi

 B.E. (Comp. Sys. Eng.)

  also known as zed
  & handle of notzed


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Sunday, 14 February 2010, 01:18

Beagle beeps

Well, some semblance of a beep, it sounds more like an alarm going off. Another frustrating tasks of mis-understanding and reboots.

To start with I had it 'click' when it turned the audio codec on or off, so at least I knew I was writing to the right registers. And changing the volume even changed the magnitude of the click.

Then after setting all the registers to something that seemed to make sense, I simply forgot to turn on the audio interface, so I spent a lot of time wondering why the McBSP end of things wasn't sending any data out (I was copying the `MP3 Out' use-case too closely which neglected to mention it ... although I read about the enable bit numerous times and knew I needed it, I simply forgot to check I was setting it). Then once I got that working I still had no sound - I couldn't get the routing to work. I knew the serial port frame signaling was at least working (no idea on the data) as changing the sample rate had an effect on how often XDRDY was asserted.

But nothing I tried worked. The audio device has quite a few registers spread all over the place, and the manual is formatted in a way which makes them hard to look up or follow. It only has a TOC entry for the whole lot, and with Evince, that makes it pretty tricky to navigate. Not to mention that Evince wants to select links half the time when you click on them just to make things more painful. I think I missed something in the manual about which serial sources receive data from the port, and data only comes in via serial 2 for non-TDM signaling. And I was trying to use serial input 1 as the source, then switching that to audio 1 internally - so I guess there was no sound from the start.

In the end I cheated a bit - I got a dump of the registers on a running GNU/Linux system which was playing sound at the time. Then I compared them one by one with those I had - on the way found some bugs anyway - and eventually just set the routing up the same way.

After all the hassles and straining to hear anything from a set of headphones I was surprised when it worked - fortunately I had the headphones off otherwise I might've got a rude shock at 3 in the morning. I'm still not using DMA to write to it, but that shouldn't be too difficult. I want to try to receive sound first before I try DMA or drop the code, but hopefully that should be simpler, since I don't have much choice in what goes where now.

Oh there was one other sting in the tail. I was playing with one of the gain controls to see if I could turn the volume down a bit: AV_ARXL2PGA. It's supposed to have a range from 0x00 to 0x3f (i.e. from mute to 0dB in 1dB steps; the master volume control), but if I set any value below about 0x30 it simply mutes the output. Which was another value I 'got wrong' when testing, so who knows, I might've had the routing correct at one point and not heard anything because of this. I'll have to have another poke at that too.

So mistakes in summary:

For the McBSP2 there was some issues too, to do with signaling polarities mostly - although I don't know if I have them right yet. Requires looking at the raw signal diagrams more closely.

Hmm, looks like a nice day for a ride.

Tagged beagle, hacking.
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