Re: [Discuss-gnuradio] Purpose of extra two bits sent to FX2 chip with data samples

[Paul Creekmore]

Eric Blossom wrote:

On Sun, Feb 01, 2009 at 12:30:58AM -0500, Paul Creekmore wrote:
  

The FPGA on USRP1 sends an extra two bits of data with each 16-bit word  
(to the FX2 chip).  If I'm not mistaken, the first bit is set when  
accompanying a sample from rx channel 0, and the second bit is set when  
accompanying an inphase (as opposed to quadrature) sample (presumably  
from channels 0, 2, 4, or 6).

* What significance do these two bits have in the handling of the data  
after the FPGA?

I'm trying to figure out how best to set these bits in the case of  
packed samples at lower quantization levels.

Thanks,
Paul
    

Uhh, what two extra bits?  The GPIF bus is 16-bits wide...

The FPGA does report two status bits (independent of the data bits)
that indicate whether (1) it has room for a new packet from the FX2
and (2) if it has a packet available for the FX2.  I don't think
anything you're doing should have anything to do with how those flags
are generated.

Did this help?

Eric
  

Hmm... no, I don't think we're talking about the same thing.  I'm, looking at usrp/fpga/sdr_lib/rx_buffer.v, lines 90 and 97.  Here's an excerpt:

88 fifo_4k_18 rxfifo
89      ( // DSP Write Side
90        .data ( {ch0_in, phase[0], fifodata} ),
91        .wrreq (~rx_full & (phase != 0)),
92        .wrclk ( rxclk ),
93        .wrfull ( rx_full ),
94        .wrempty ( ),
95        .wrusedw ( ),
96        // USB Read Side
97        .q ( {ch0_out,iq_out,usbdata} ),
98        .rdreq ( RD & ~read_count[8] ),
99        .rdclk ( ~usbclk ),
100       .rdfull ( ),
101       .rdempty ( ),
102       .rdusedw ( rxfifolevel ),
103       // Async, shared
104       .aclr ( reset ) );

This is the port mapping for the receive fifo.  The fifo is 18 bits wide, and "ch0_in" and "phase[0]" are concatenated with the 16-bit data word to form the input to the fifo.

--Paul