Invert Pk and Ik if needed

Author: PaulZC

src/SparkFun_SiT5358.cpp

@@ -275,6 +275,7 @@ bool SfeSiT5358Driver::setFrequencyByBiasMillis(double bias, double Pk, double I
 
     double clockInterval_s = 1.0 / freq; // Convert freq to interval in seconds
 
+    // bias (RXClkBias, milliseconds) is the error term we are trying to drive to zero
     double biasInClocks = bias / 1000.0; // Convert bias from millis to seconds
     biasInClocks /= clockInterval_s; // Convert bias to clock cycles
 
@@ -293,11 +294,17 @@ bool SfeSiT5358Driver::setFrequencyByBiasMillis(double bias, double Pk, double I
             biasInClocks = 0.0 - maxChangeInClocks;
     }
 
+    // If RxClkBias is positive, we need to reduce the oscillator frequency by making
+    // the frequency control word more negative. Both Pk and Ik must be negative.
+    if (Pk > 0.0)
+        (Pk *= -1.0);
+    if (Ik > 0.0)
+        (Ik *= -1.0);
     double P = biasInClocks * Pk;
     double dI = biasInClocks * Ik;
-    I += dI;
+    I += dI; // Add the delta to the integral
 
-    return setFrequencyHz(P + I);
+    return setFrequencyHz(P + I); // Set the frequency to proportional plus integral
 }
 
 /// @brief Convert the 4-bit pull range into text

src/SparkFun_SiT5358.h

@@ -191,14 +191,6 @@ class SfeSiT5358Driver
     void setMaxFrequencyChangePPB(double ppb);
 
 
-    /// @brief Set the frequency according to the GNSS receiver clock bias in milliseconds
-    /// @param bias the GNSS RX clock bias in milliseconds
-    /// @return true if the write is successful
-    /// Note: the frequency change will be limited by: the pull range capabilities of the device;
-    ///       and the setMaxFrequencyChangePPB. Call getFrequencyHz to read the frequency set.
-    bool setFrequencyByBiasMillis(double bias);
-
-
     /// @brief Set the frequency according to the GNSS receiver clock bias in milliseconds
     /// @param bias the GNSS RX clock bias in milliseconds
     /// @param Pk the Proportional term