Hi, all.

I am building an interface between an ADSP2181 SPORT (serial port) and an
RS232.  I have been able to make the DSP talk SPI before, so I thought a
fast PIC may be able to do the conversion.  Selected PIC for this project is
an 'F877.  I had hoped to spurt SPI data out as a very fast stream,
concurrently building the packet frame, then resend it as a more or less
continuous asynchronous stream.  The problem gets further complicated
because the RS232 is at 220.5kbaud - fast and non standard.  That problem is
solved by judicious selection of the crystal - either 14.318MHz or 18MHz.

The DSP is a battery powered data acquisition system interrupt driven by an
ADC.  Data is continuous and not all the SPORT handshake lines are
available.  The DSP does some pretty serious filtering before passing the
data so tasks are already pretty tight and optimized: get data, filter, send
data.  It already works and works well - 1 bit in 20 noise after a gain of
103 (like I said, lots of filtering):-).

My problem is this:  Using PICC, I have written simple minded code to
generate data so I can see if the PIC can keep up.  RS232 seems to be able
to, but [[[I have seen very long latencies between SPI words (operating at
CLK/4)]]].  Is this normal?  If these can be shortened, how and how much?  I
have browsed archives and am starting the process of understanding the
compiler listing to see if there is something obvious.

I have only been browsing here for a week or so and have been pleased to see
that this group seems to have a lot of good idea people.  I'm new to PICC,
don't know it well, but suspect that this project will need to have very
tight control over the interrupt handling and may be better done in
assembly.  Although a single chip solution is preferred from a power
standpoint (and there needs to be some smarts with the RS232
initialization), I can add a small, low power FPGA or serial to parallel
shift registers.  Any suggestions for the small FPGA?

Sorry about the rambling...

Ed Browne

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