Build Instructions

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• 1

  Wiring

  Wiring is:

  • Black - Ground
  • White -  to RX on the Arduino
  • Green - to TX on the Arduino
  • Red - to 5V (3.3v should also do).

  The green and yellow arrows are the test points as shown in the Oscilloscope screenshots.

  If you want to embed it into a project, you can either solder a lead to the pad marked by the yellow arrow, or unsolder the Led and the resistor and solder a lead across the resistor pads.

• 2

  Sketch

  The Arduino sketch is pretty simple.

  In the Library Manager install "SparkFun u-blox GNSS Arduino Library".

  The sketch code (much of it is from the SparkFun Github examples):

  #include <SparkFun_u-blox_GNSS_Arduino_Library.h>
  
  // desired frequency
  #define FREQ 12000000
  // serial for communicating with the host, on Arduino Zero
  #define HOST_SERIAL SerialUSB
  //serial for pins D0/D1 on Arduino Zero
  #define PER_SERIAL Serial1
  
  SFE_UBLOX_GNSS myGNSS;
  
  void setup() {
    // put your setup code here, to run once:
   PER_SERIAL.begin(9600);
  
    delay(3000);
    if (!myGNSS.begin(PER_SERIAL)){
      HOST_SERIAL.println("\nFailed connecting to GPS module.");
    } else {
      HOST_SERIAL.println("GPS Module connected.");
  
      HOST_SERIAL.print(F("Version: "));
      byte versionHigh = myGNSS.getProtocolVersionHigh();
      HOST_SERIAL.print(versionHigh);
      HOST_SERIAL.print(".");
      byte versionLow = myGNSS.getProtocolVersionLow();
      HOST_SERIAL.println(versionLow);
  
      setPPS();
    }
  }
  
  void setPPS() {
    // Create storage for the time pulse parameters
    UBX_CFG_TP5_data_t timePulseParameters;
  
    // Get the time pulse parameters
    if (myGNSS.getTimePulseParameters(&timePulseParameters) == false)
    {
      HOST_SERIAL.println(F("getTimePulseParameters failed! Freezing..."));
      while (1) ; // Do nothing more
    }
  
    // Print the CFG TP5 version
    HOST_SERIAL.print(F("UBX_CFG_TP5 version: "));
    HOST_SERIAL.println(timePulseParameters.version);
  
    timePulseParameters.tpIdx = 0; // Select the TIMEPULSE pin
    //timePulseParameters.tpIdx = 1; // Or we could select the TIMEPULSE2 pin instead, if the module has one
  
    // We can configure the time pulse pin to produce a defined frequency or period
    // Here is how to set the frequency:
  
    // While the module is _locking_ to GNSS time, make it generate 5Hz
    timePulseParameters.freqPeriod = 5; // Set the frequency/period to 5Hz
    timePulseParameters.pulseLenRatio = 0x55555555; // Set the pulse ratio to 1/3 * 2^32 to produce 33:67 mark:space
  
    // When the module is _locked_ to GNSS time, make it generate select FREQ
    timePulseParameters.freqPeriodLock = FREQ; // Set the frequency/period
    timePulseParameters.pulseLenRatioLock = 0x80000000; // Set the pulse ratio to 1/2 * 2^32 to produce 50:50 mark:space
  
    timePulseParameters.flags.bits.active = 1; // Make sure the active flag is set to enable the time pulse. (Set to 0 to disable.)
    timePulseParameters.flags.bits.lockedOtherSet = 1; // Tell the module to use freqPeriod while locking and freqPeriodLock when locked to GNSS time
    timePulseParameters.flags.bits.isFreq = 1; // Tell the module that we want to set the frequency (not the period)
    timePulseParameters.flags.bits.isLength = 0; // Tell the module that pulseLenRatio is a ratio / duty cycle (* 2^-32) - not a length (in us)
    timePulseParameters.flags.bits.polarity = 1; // Tell the module that we want the rising edge at the top of second. (Set to 0 for falling edge.)
  
    // Now set the time pulse parameters
    if (myGNSS.setTimePulseParameters(&timePulseParameters) == false)
    {
      HOST_SERIAL.println(F("setTimePulseParameters failed!"));
    }
    else
    {
      HOST_SERIAL.println(F("Success!"));
    }
  }
  
  void loop() {
    
    delay(1000);
    
  }

   A few things that you can just change out of the box:

  FREQ - set to the desired frequency.

  HOST_SERIAL - the serial to use for communicating to the host (meaning your computer).

  PER_SERIAL - the serial for communicating to the GPS module. 

  The last two you might need to change depending on the flavour of your Arduino. The #defines are set for Arduino Zero (Adafruit Metro M0 if to be precise).

  And the expected output is:

  GPS Module connected.
  Version: 18.0
  UBX_CFG_TP5 version: 1
  Success!
  

  Your Red LED should be flickering at a 5Hz frequency until a GPS lock is acquired. After that it should change to constant red if your FREQ frequency is high enough (probably over 50Hz).

• 3

  Enjoy

  Enjoy a Reference Frequency that you can use either to calibrate a frequency counter, or use it as a oscillator for your project.

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