Re: [Discuss-gnuradio] Low speed ADC-DAC from Python ?

[Eric Blossom]

On Mon, Apr 03, 2006 at 06:34:24AM -0700, Angilberto Muniz Sb wrote:
> Folks,
> Is it possible to read/write from/to the low speed
> ADC/DAC from Python?

The python interfaces to the usrp are specified in
gr-usrp/src/usrp1.py.  Check usrp/host/lib/usrp_basic.h for the
missing documentation (sorry about that.  We've got a situation that's
violating the "write once" rule):


  //! magic value used on alternate register read interfaces
  static const int READ_FAILED = -99999;

  /*!
   * \brief Write auxiliary digital to analog converter.
   *
   * \param which_dboard        [0,1] which d'board
   *                            N.B., SLOT_TX_A and SLOT_RX_A share the same 
AUX DAC's.
   *                            SLOT_TX_B and SLOT_RX_B share the same AUX 
DAC's.
   * \param which_dac           [2,3] TX slots must use only 2 and 3.
   * \param value               [0,4095]
   * \returns true iff successful
   */
  bool write_aux_dac (int which_board, int which_dac, int value);

  /*!
   * \brief Read auxiliary analog to digital converter.
   *
   * \param which_dboard        [0,1] which d'board
   * \param which_adc           [0,1]
   * \returns value in the range [0,4095] if successful, else READ_FAILED.
   */
  int read_aux_adc (int which_dboard, int which_adc);


So from python:

  d_board = 0   # 0: side A, 1: side B

  v = u.read_aux_adc(d_board, 0)
  ok = write_aux_dac(d_board, 2, 2048)

Be sure to pay attention to the comments above that start with N.B.
You may also want to take a look at the usrp schematics if this isn't clear.


> If possible, what would be a safe rate ?

It's pretty slow.  You'll need to try it to see how fast it'll run on
your system.  Each read or write is a round trip across the USB, plus
bit banging the SPI bus to talk to the AD9862.  There's no clock
pacing the reads and writes of the the aux dac/adc's, so the samples
won't be evenly spaced.  Most daughterboards that use them, use them
for things like setting an analog gain control, or reading back an RSSI
indication.

Eric