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Re: [Discuss-gnuradio] FrontEnd4702 take 29

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] FrontEnd4702 take 29
Date: Wed, 25 Feb 2004 15:29:46 -0800
User-agent: Internet Messaging Program (IMP) 4.0-cvs

Quoting Eric Blossom <address@hidden>:

> 4702 I2C Whackiness
> -------------------
> The 4702 can decode at 4 different addresses, but does it though one
> pin.  Matt will work up a simple circuit that uses the I2C_A0 and A1
> bits as input and drives the magic pin correctly.  This will ensure
> that the 4702 board needs no jumpers and will perform properly in any
> RX slot.

Here goes:

A0 Tells you whether this is a TX or an RX board.  0 = TX, 1 = RX.  Since we are
only talking about RX boards for the 4702, this won't matter.

A1 Tells you whether you are in the first or second TX/RX pair.  A1=0  means you
are on side A, A1=1 means side B.  This is the one we are interested in.

The simplest way to control the addresses on the 2 4702s on each board is this:

On one of the 4702s, the Address select line should be tied to the drain of an
N-MOS transistor.  The source of the transistor should go to ground.  The gate
goes to A1.  Thus, when A1 = 0 (i.e. Unit A), the AS line will be OPEN.  When
A1 = 1 (i.e. Unit B), the AS line will be Grounded.

On the other 4702, the AS line should go to a 15 K resistor to +5V.  There
should be the source and drain of a P-MOS transistor in parallel with it.  When
the P-MOS is off, AS is in the "15K" configuration.  When the P-MOS is on, AS
is at +5V.  The gate of the P-MOS is connected to both a 10K resistor to +5V
and to the drain of an N-MOS.  The P-MOS is controlled by the drain of an
N-MOS.  This second N-MOS should have its source at ground, and its gate should
connect to A1.

This covers all 4 addresses for the 4702, there are no jumpers, everything is
automatic when you plug it in, it only takes 2 resistors, 2 N-MOS transistors,
and one P-MOS.  You can use the cheapest, lowest current, slowest transistors
you can find, as long as they have a Vds higher than 5V.  I hope everyone can
be happy now.


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