Re: [Discuss-gnuradio] Using USRP w/o Daughterboard's

[Eric Blossom]

On Fri, Nov 11, 2005 at 12:02:18PM -0500, address@hidden wrote:
> No I have not yet made that change.  I have been trying to figure out how
> to use it.  I don't understand the brief description
> 
>   /*!
>    * \brief Enable/disable automatic DC offset removal control loop in FPGA
>    *
>    * \param bits  which control loops to enable
>    * \param mask  which \p bits to pay attention to
>    *
>    * If the corresponding bit is set, enable the automatic DC
>    * offset correction control loop.
>    *
>    * <pre>
>    * The 4 low bits are significant:
>    *
>    *   ADC0 = (1 << 0)
>    *   ADC1 = (1 << 1)
>    *   ADC2 = (1 << 2)
>    *   ADC3 = (1 << 3)
>    * </pre>
>    *
>    * By default the control loop is enabled on all ADC's.
>    */
>
>  bool set_dc_offset_cl_enable(int bits, int mask);


> I don't know what to use for bit or mask values.  Is this like the mux
> values? Please explain how you determine the bit and mask values to use?


Hi Mike,

Many places in the code we use a pair of arguments with names like
"bits and mask" or "val and mask".  This pattern allows us modify only
a subset of the bits in a given target word or register.

Assuming that "reg" is the value we are modifying, the result is
equivalent to

    reg = (reg & ~mask) | (bits & mask)

We use this pattern because it allows us to atomically implement the
read/modify/write on for example the FPGA, where it's safe even in the
face of multiple processes trying to modify different bits in the same
register.

Eric