[hpsdr] ALEX - Call for Comments - II

[Steven Wilson]

Philip Covington wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> On 3/6/07, Graham Haddock <grahamh at verizon.net> wrote:
> <snip>
>   
>> 2.) Control...
>> This is a receiver front end preselector.  I would prefer
>> to not have any in-band oscillators, clocks, or noisy high speed
>> buses appear on the card.  For that reason I would propose that
>> this be a "dumb" slave card, controlled by a three wire SPI bus,
>> which would be filtered upon entrance.  I2C would also work.
>> Phil H wanted I2C, I personally prefer SPI, Lyle seems to like SPI.
>>     
>
> As soon as you run SPI or I2C to the board you have a clock signal on
> your board.  You don't need an on board oscillator when using the CPLD
> so it is equivalent to using a dedicated SPI or I2C expander chip in
> that regard.  I believe the CPLD's output slew rate can be programmed
> to reduce noise also.
>   

This isn't really true with I2C - you have to provide for deglitching
the signal - I2C says you have to be able to handle a 50ns glitch. This
is done most easily with a series of flops running at some much faster
frequency used to smooth out the glitches - means I2C needs an on-board
clock. 

There might be a passive filtering solution possible here - I'm only
familiar with the digitial way of providing the function.

Steve KA6S

< Snip>
> Phil N8VB
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