Re: [Discuss-gnuradio] FrontEnd4702 take 2

[Eric Blossom]

On Tue, Feb 24, 2004 at 11:36:35PM -0800, John Gilmore wrote:
> > >     There are now two 4702 tuners on the board. The first one has its MA0
> > > pin grounded so it should respond to I2C address 0xC0 and the second has 
> > > its
> > > MA0 pin left open so it should respond to I2C address 0xC2.
> > 
> > While we're at it, let's really go all out -- the 4702 can handle 4 
> > different
> > addresses.  Thus, we could put 2 of these boards, with 2 4702s each, on the
> > USRP at once, necessitating the 4 addresses.  With a pair of 2-pin jumpers 
> > we
> > can accomodate that.  If you give the address select pin on one 4702 a 15K 
> > R to
> 
> I thought these daughterboards are physically addressable.

They are.  The daughterboard ought to look at the I2C_A0 and A1 bits
and do the right thing.  

This still leaves a snafu regarding using the aux_DAC outputs for AGC,
since there aren't enough of them for each card to have it's own, let
alone two.

The Analog Devices AD8801 TrimDAC would do the job.  It's kind of
overkill, but it would work.  It's got 8 8-bit DACs in it, and runs on
the 3 wire SPI bus at 3.3V.  You'd still need the single-supply
rail-to-rail op-amp at +5V to get the range right.  The daughtercards
each support SPI with board specific enables.  There are probably
similar parts from other vendors.

> Doing something like this, to allow many copies of a board to be
> plugged in at once without trouble, is a good idea for any board.  It
> makes the system simple for software, simple for users, and simple for
> hardware builders.

I heartily agree.  We ought to be able to plug the daughterboards in
to any slot without setting jumpers, and they ought to work.

>       John

Eric