[LAU] open hw soundcard

[Folderol]

On Mon, 16 Nov 2009 10:56:17 +0100 (CET)
karl at aspodata.se (Karl Hammar) wrote:

> Martin Peach:
> > >> karl at aspodata.se (Karl Hammar) wrote:
> ...
> > >>> If we are going to have multiple analog inputs at higt sample rate,
> > >>> isn't it better to have a parallell interface. With spi the number of
> > >>> channels will be limited to something like 8 for a 24bit converter.
> > >>> Plus that the AT32AP7000/AT91SAM9260 only has two spi-busses.
> ...
> > Another way to do that would be to bit-bang 8 SPI devices sharing one 
> > clock line (and /CS/FS) onto a byte-wide port, read all 8 in parallel 
> > one bit at a time in perfect sync.
> > 
> > It actually should be possible to use a /RD line for an SPI clock, so 24 
> > successive reads of a single address would clock in 8 24-bit samples.
> 
> I tend to dislike bitbanging, but yes, that should be doable, something
> like:
> 
> void read_all_adcs() {
>   uint32_t adc[32];  // up to 32 adc converters
>   int ix;
>   int kx;
>   uint32_t bits;
> 
>   chip_select(0);
>   // the first few bits are to start converter
>   bits = get_data();    
>   bits = get_data();    
>   bits = get_data();    
>   bits = get_data();    
>   bits = get_data();    
>   for (ix = 0; ix < 24; ix++) {
>     bits = get_data();    
>     for (kx = 0; kx < 32; kx++) {
>       adc[kx] <<= 1;
>       adc[kx] |= bits & 1;
>       bits >>= 1;
>     }
>   }
>   chip_select(1);
> }
> 
> I have to think about that. The big "but" would be that the clock
> (the frequency of get_data()) should be stable, I don't know how much
> variation is allowed.
> 
> Regards,
> /Karl

Possibly, the only one that needs to be stable is the one that actually
initiates the sampling. i.e. number 5. This seems a bit messy, however,
could numbers 1-4 be done at any time after the previous conversion,
then number 5 initiated by a very stable hardware clock source, and the
remaining pulses derived from the processor.

indeed, turn it round so:

   chip_select(1); // end of previous cycle
// some form of minimal time delay
   chip_select(0);
   // the first few bits are to start converter
   bits = get_data();    
   bits = get_data();    
   bits = get_data();    
   bits = get_data();    
// wait in this state for hardware clock

Because we've faked a serial fetch would that upset the normal fetch, or
can we simply make one more than we need and throw it away?


The hardware clock source could also be used to transmit the
syncronising pulse to the network - assuming this card is the master,
which the first development one will be of course.


Hmm, we now have a big enough thread for me to create a new fold in my
mailreader just for these entries :)

-- 
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.