Re: [Discuss-gnuradio] digital noise input

[Eric Blossom]

On Fri, Nov 10, 2006 at 06:01:02PM -0800, Oussama Sekkat wrote:
> Hi,
> 
> I'm trying to use my logic analyzer to measure the received noise.
> Here is how I proceed to do that. First in the verilog code, I connect the
> 12 bit rx_a_a  ADC input to the io_rx_a DEBUG output by adding this line of
> code:
> assign io_rx_a = {4'b0000, rx_a_a }

A much less invasive way to handle this would have been to take
advantage of the support for sending whatever debug values you want to
the i/o pins by using the debug_0,..., debug_3 parameters of the
master_control block.  See also the FR_DEBUG_EN register documented
in fpga_regs_common.{h,v}

// If the corresponding bit is set, internal FPGA debug circuitry
// controls the i/o pins for the associated bank of daughterboard
// i/o pins.  Typically used for debugging FPGA designs.

#define FR_DEBUG_EN             14
#  define bmFR_DEBUG_EN_TX_A           (1 << 0)        // debug controls TX_A 
i/o
#  define bmFR_DEBUG_EN_RX_A           (1 << 1)        // debug controls RX_A 
i/o
#  define bmFR_DEBUG_EN_TX_B           (1 << 2)        // debug controls TX_B 
i/o
#  define bmFR_DEBUG_EN_RX_B           (1 << 3)        // debug controls RX_B 
i/o


> I then load the .rbf file to the FPGA, connect the io_rx_a outputs of the
> LFRX daughter board to the logic analyzer and observe what I get. I make
> sure that no antenna is connected to the RX-A port.
> To my surprise, some of the upper bits toggle. For example, bit 11, 10, 9,
> 8, 5 ,4 , 3 ,2 ,1 0, toggle. Since I should only be getting noise, I was
> expecting to get a very low signal where only the least significant bit
> would be switiching between low and high values.

Are they being driven or are they just floating?

My guess is that they are floating.

My suggestion (once again) is to use some of debug_0,... debug_3,
write to FR_DEBUG_EN and then enable the outputs of the particular
pins using u._write_oe(which, value, mask)

> Does anyone know what could be the origin of such results? Any insight would
> be greatly appreciated.
>
> Thank you,
> 
> Oussama.

Eric