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Re: [Discuss-gnuradio] understanding writing to registers

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] understanding writing to registers
Date: Mon, 13 Aug 2007 11:29:43 -0700
User-agent: Mutt/1.5.9i

On Mon, Aug 13, 2007 at 02:01:05PM -0400, George Nychis wrote:
> Okay, so we're trying to get read/write registers to work correctly, and 
> we're trying to understand how a register read/write currently works 
> using the following methods:
>   _write_fpga_reg()
>   _write_oe()

If you trace through the code, you'll see that _write_oe ends up
calling _write_fpga_reg.

> We are monitoring the serial I/O enable signal using a logic analyzer 
> and writing to registers using the top two methods.  When using them, we 
> never see the serial I/O enable signal asserted like we think it should. 
> So we're wondering, what happens when these methods are called?

I'm not sure why you're looking at any of the serial buses at this
point.  They are completely unrelated to anything having to do with
writing an FPGA register using inband signaling ;)

Also, after face-to-face discussion with Matt over the past few days,
it turns out that we will not need to implement either the I2C or SPI
interfaces in the FPGA for the USRP1.

> Second, we are trying to re-write serial i/o to support the read/write 
> C/S packets but do not understand serial i/o that well.  We don't quite 
> understand the input and output.  Can anyone explain it at all, so we 
> understand what we need to change about it and what our new design is 
> going to look like?

Summary: don't worry about SPI or I2C i/o from the FPGA.

To control the PLL and rx gain on the RFX-* boards on the USRP1, we'll
move some resistors on the RFX boards, enabling an alternate control
mechanism via the daughterboard i/o pins.  More on that later.

> Thanks!
> George


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