> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Richard Seriani, Sr.
> Sent: 22 January 2008 11:33
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] Audio ADC-DAC Problems
> 
> Mike,
> 
> According to the 25LC1024 datasheet, the internal write cycle time
(byte
> or
> page) is 6ms (max). I am doing page writes, so I load 256 bytes of
> converted
> data into the EEPROM input buffer, bring !CS high to initiate the
internal
> write, then poll the status register WIP bit to check that the write
is
> complete.
> 
> Hmm, this means I am losing somewhere around 6ms of input for each 256
> bytes
> I am recording. Quick math shows that if I record about 31.25ms of
audio
> for
> each 256 bytes, I have about a 20% hole in the data. Does that sound
> right?
> 
> Okay, sounds like I need to look at some sort of double-buffering of
the
> a-d
> data? I think I may be able to load the 25LC1024 input buffer while
the
> array write is taking place, but I need to check that. If so, that may
be
> one solution that could be implemented simply by shifting a few
> instructions
> in my code. I would load the buffer, then check the WIP bit to be sure
the
> write to the array is complete before I initiate the next write. The
write
> sould be complete since the conversion of 256 bytes will take about 5
> times
> as long as the EEPROM write cycle, so I may be able to get away
without
> checking it, though I am not sure I would want to.
> 
> Thanks for making me think about this.

I'd be very surprised if you can write to the buffer during a write
operation.  Serial EEPROMs with SPI interfaces generally only let you
read the status register during a write cycle, but the datasheet will
certainly tell you one way or the other.

However, all is not lost.  6ms at 8Khz is only 48 samples.  Simply
implement a circular buffer (a little larger than 48 samples for
safety), with the ADC inserting data into the buffer, and the SPI taking
data out.  Whilst the EEPROM is not writing the buffer will never be
more than one byte full assuming you have a reasonable SPI clock rate.
Once the page write commences the ADC will start filling up the buffer,
but once full the SPI will be able to empty it very quickly.

If available, use the CCP modules special event trigger to take ADC
conversions at a regular rate, and use the ADC interrupt to put data
into the buffer, and the SPI interrupt to take data out.  The whole
recording process then runs totally under interrupt control, so you can
carry on doing stuff in your main loop.

Regards

Mike

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