Wiring for Arduino

You can easily wire this breakout to any microcontroller, we'll be using an Arduino. For another kind of microcontroller, just make sure it has I2C capability, then port the code - its pretty simple stuff!


  • Connect Vin to the power supply, 3-5V is fine. Use the same voltage that the microcontroller logic is based off of. For most Arduinos, that is 5V. Feathers, ItsyBitsys, etc. use 3V. Check your board documentation.
  • Connect GND to common power/data ground
  • Connect the SCL pin to the I2C clock SCL pin on your Arduino. On an UNO & '328 based Arduino, this is also known as A5, on a Mega it is also known as digital 21 and on a Leonardo/Micro, digital 3
  • Connect the SDA pin to the I2C data SDA pin on your Arduino. On an UNO & '328 based Arduino, this is also known as A4, on a Mega it is also known as digital 20 and on a Leonardo/Micro, digital 2

Download Adafruit_Si5351

To begin reading sensor data, you will need to download the Adafruit Si5351 library from the Arduino library manager.

Open up the Arduino library manager:

Search for the Adafruit Si5351 library and install it

We also have a great tutorial on Arduino library installation at:

Load Demo Sketch

Now you can open up File->Examples->Adafruit_Si5351->Si5351 and upload to your Arduino wired up to the sensor

Then open up the Serial console at 9600 baud to check the output. You should see the following:
Now you can use your oscilloscope to probe the #0, #1 and #2 outputs
Depending on your oscillioscope make and model, it may not be possible for you to verify the 112.5MHz output frequency!
That's it! If you want to change the frequencies, adjust the example sketch and re-upload.

Library Reference

The library we have is simple and easy to use

You can create the Adafruit_Si5351 object with:

Adafruit_SI5351 clockgen = Adafruit_SI5351();

I2C does not have pins, as they are fixed in hardware.


To initialize the chip, call clockgen.begin() which will check that it can be found. Begin() returns true/false depending on these checks. We suggest you wrap begin() in a statement that will check if the chip was located:

  if (clockgen.begin() != ERROR_NONE)
    /* There was a problem detecting the IC ... check your connections */
    Serial.print("Ooops, no Si5351 detected ... Check your wiring or I2C ADDR!");

Set up the PLL

The chip uses two subsections to generate clock outputs. First it multiplies the 25MHz reference clock by some amount (setting up the PLL), then it divides that new clock by some other amount (setting up the clock divider)

By noodling with the multiplier and divider you can generate just about any clock frequency!

There are two PLL multipliers (A and B), so if you want to have three outputs, two outputs will have to share one PLL.

Set up the PLL with 'integer mode'

The cleanest way to run the PLL is to do a straight up integer multiplication:

clockgen.setupPLLInt(SI5351_PLL_A or SI5351_PLL_B, m);

This sets PLL_A or PLL_B to be 25MHz * m and m (the integer multiplier) can range from 15 to 90!

Set up the PLL with 'fractional mode'

This mode allows a much more flexible PLL setting by using fractional multipliers for the PLL setup, however, the output may have a slight amount of jitter so if possible, try to use integer mode!
clockgen.setupPLLInt(SI5351_PLL_A or SI5351_PLL_B, m, n, d);

This sets PLL_A or PLL_B to be 25MHz * (m + n/d)

  • m (the integer multiplier) can range from 15 to 90
  • n (the numerator) can range from 0 to 1,048,575
  • d (the denominator) can range from 1 to 1,048,575

Set up the clock divider

Once you have the PLLs set up, you can now divide that high frequency down to get the number you want for the output

Each output has its own divider. You can use the cleaner Integer-only divider:

clockgen.setupMultisynthInt(output, SI5351_PLL_x, SI5351_MULTISYNTH_DIV_x);
  • For the output use 0, 1 or 2
  • For the PLL input, use either SI5351_PLL_A or SI5351_PLL_B
  • For the divider, you can divide by SI5351_MULTISYNTH_DIV_4, SI5351_MULTISYNTH_DIV_6, or SI5351_MULTISYNTH_DIV_8

Again, integer output will give you the cleanest clock. If you need more flexibility, use the fractional generator/divider:

clockgen.setupMultisynth(output, SI5351_PLL_x, div, n, d);
  • For the output use 0, 1 or 2
  • For the PLL input, use either SI5351_PLL_A or SI5351_PLL_B
  • The final frequency is equal to the PLL / (div + n/d)
  • div can range from 4 to 900
  • n can range from 0 to 1,048,575
  • d can range from 1 to 1,048,575

Additional R Divider

If you need to divide even more, to get to the < 100 KHz frequencies, there's an additional R divider, that divides the output once more by a fixed number:

clockgen.setupRdiv(output, SI5351_R_DIV_x);

output is the clock output #
The R divider can be any of the following:

  • SI5351_R_DIV_1
  • SI5351_R_DIV_2
  • SI5351_R_DIV_4
  • SI5351_R_DIV_8
  • SI5351_R_DIV_16
  • SI5351_R_DIV_32
  • SI5351_R_DIV_64
  • SI5351_R_DIV_128


As you can see, the annoying part here is figuring out the best choice for PLL multipler & divider! SiLabs has a desktop application called ClockBuilder that can do some calculation of the PLL divider/multiplier for you. It's Windows only, but you only need to use it once for calculation.

Install and run, select the Si5351A with 3 outputs, and Do not connect to the EVB

Enable the output you want, and set the frequency as floating point or fraction
Set up the crystal to be 25 MHz (the default is 27 MHz)
Click on Create Frequency Plan to see the PLL and divider setups!

Earlier versions of this chip only take a divider of 900 or less, and our library doesn't let you select > 900 for the integer div. So if you get a higher value from the calculator, you may need to adjust it!

This guide was first published on Aug 12, 2014. It was last updated on Nov 28, 2023.

This page (Arduino Code) was last updated on Aug 12, 2014.

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