RFNoC: Fix DSP frequency accuracy

The host code was calculating and programming a 32-bit value for the DSP
frequency, but the DDS modules in the FPGA only use the upper 24-bits.
This led to inaccurate frequency values being returned.  This change
corrects the resolution of the value on the host side so an accurate
value is returned.

Signed-off-by: michael-west <michael.west@ettus.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/host/lib/include/uhdlib/usrp/cores/dsp_core_utils.hpp b/host/lib/include/uhdlib/usrp/cores/dsp_core_utils.hpp
index 74b4c3848..6371eb2e5 100644
--- a/host/lib/include/uhdlib/usrp/cores/dsp_core_utils.hpp
+++ b/host/lib/include/uhdlib/usrp/cores/dsp_core_utils.hpp
@@ -9,16 +9,19 @@
 
 #include <tuple>
 
-/*! For a requested frequency and sampling rate, return the
- *  correct frequency word (to set the CORDIC) and the actual frequency.
+/*! For a requested frequency, sampling rate, and frequency word width (in
+ *  number of bits), return the correct frequency word (to set the CORDIC or
+ *  DDS) and the actual frequency.
  */
 void get_freq_and_freq_word(const double requested_freq,
     const double tick_rate,
     double& actual_freq,
-    int32_t& freq_word);
+    int32_t& freq_word,
+    int word_width = 32);
 
-/*! For a requested frequency and sampling rate, return the
- *  correct frequency word (to set the CORDIC) and the actual frequency.
+/*! For a requested frequency, sampling rate, and frequency word width (in
+ *  number of bits), return the correct frequency word (to set the CORDIC or
+ *  DDS) and the actual frequency.
  */
 std::tuple<double, int> get_freq_and_freq_word(
-    const double requested_freq, const double tick_rate);
+    const double requested_freq, const double tick_rate, int word_width = 32);