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Si5351 Library for Arduino

This is a basic library for the Si5351 series of clock generator ICs from Silicon Labs for the Arduino development environment. It will allow you to control the Si5351 with an Arduino, and without depending on the proprietary ClockBuilder software from Silicon Labs.

This library is focused towards usage in RF/amateur radio applications, but it may be useful in other cases. However, keep in mind that coding decisions are and will be made with those applications in mind first, so if you need something a bit different, please do fork this repository. Also, since the Si5351A3 version is the one which seems most useful in amateur radio applications, this is where the current development will be focused. Once the Si5351A3 has a decent and mature feature set, hopefully we will be able to turn to the 8-output version, and perhaps even the B and C variants.

Please feel free to use the issues feature of GitHub if you run into problems or have suggestions for important features to implement.

Thanks For Your Support!

If you would like to support my library development efforts, I would ask that you please consider purchasing a Si5351A Breakout Board from my online store at etherkit.com. Thank you!

Library Installation

The best way to install the library is via the Arduino Library Manager, which is available if you are using Arduino IDE version 1.6.2 or greater. To install it this way, simply go to the menu Sketch > Include Library > Manage Libraries..., and then in the search box at the upper-right, type "Etherkit Si5351". Click on the entry in the list below, then click on the provided "Install" button. By installing the library this way, you will always have notifications of future library updates, and can easily switch between library versions.

If you need to or would like to install the library in the old way, then you can download a copy of the library in a ZIP file. Download a ZIP file of the library from the GitHub repository by using the "Download ZIP" button at the right of the main repository page. Extract the ZIP file, then rename the unzipped folder as "Si5351". Finally, open the Arduino IDE, select menu Sketch > Import Library... > Add Library..., and select the renamed folder that you just downloaded. Restart the IDE and you should have access to the new library.

Hardware Requirements and Setup

This library has been written for the Arduino platform and has been successfully tested on the Arduino Uno and an Uno clone. There should be no reason that it would not work on any other Arduino hardware with I2C support.

The Si5351 is a +3.3 V only part, so if you are not using a +3.3 V microcontroller, be sure you have some kind of level conversion strategy.

Wire the SDA and SCL pins of the Si5351 to the corresponding pins on the Arduino. Use the pin assignments posted on the Arduino Wire library page. Since the I2C interface is set to 100 kHz, use 1 to 10 kΩ pullup resistors from +3.3 V to the SDA and SCL lines.

Connect a 25 MHz or 27 MHz crystal with a load capacitance of 6, 8, or 10 pF to the Si5351 XA and XB pins. Locate the crystal as close to the Si5351 as possible and keep the traces as short as possible. Please use a SMT crystal. A crystal with leads will have too much stray capacitance.


Include the Si5351Arduino library into your instance of the Arduino IDE. Download a ZIP file of the library from the GitHub repository by using the "Download ZIP" button at the right of the main repository page. Extract the ZIP file, then rename the unzipped folder as "Si5351". Finally, open the Arduino IDE, select menu Sketch > Import Library... > Add Library..., and select the renamed folder that you just downloaded. Restart the IDE and you should have access to the new library.

(Yes, the Arduino IDE can import a ZIP file, but it doesn't like filenames with characters such as dashes, as GitHub does when it appends the branch name with a dash. Perhaps there's an elegant way around this, we'll see.)

There is a simple example that is placed in your examples menu under Si5351Arduino. Open this to see how to initialize the Si5351 and set a couple of the outputs to different frequencies. The commentary below will analyze the sample sketch.

Before you do anything with the Si5351, you will need to include the "si5351.h" and "Wire.h" header files and instantiate the Si5351 class.

#include "si5351.h"
#include "Wire.h"

Si5351 si5351;

Now in the Setup() function, let's initialize communications with the Si5351, specify the load capacitance of the reference crystal, and to use the default reference oscillator frequency of 25 MHz:

si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0);

Next, let's set the CLK0 output to 14 MHz and use a fixed PLL reference frequency (this allows for glitch-free tuning):

si5351.set_pll(SI5351_PLL_FIXED, SI5351_PLLA);
si5351.set_freq(1400000000ULL, SI5351_PLL_FIXED, SI5351_CLK0);

Frequencies are indicated in units of 0.01 Hz. Therefore, if you prefer to work in 1 Hz increments in your own code, simply multiply each frequency passed to the library by 100ULL (better yet, use the define called SI5351_FREQ_MULT in the header file for this multiplication). The second value passed in the above method is the desired driving PLL frequency. Entering a 0 will have the method choose a PLL frequency for you. If you would like to use a fixed PLL frequency to drive a multisynth (in order to ensure glitch-free tuning), set the desired PLL frequency first using the method below, then specify that frequency in the set_freq() method. The PLL frequency only needs to be set once. Any additional frequency changes only need to use the set_freq() method as long as you are using the same PLL frequency as before.

Now let's set the CLK1 output to 20 MHz output, and let the Si5351 class pick a PLL frequency:

si5351.set_freq(2000000000ULL, 0ULL, SI5351_CLK1);

In the main Loop(), we use the Serial port to monitor the status of the Si5351, using a method to update a public struct which holds the status bits:

Serial.print("SYS_INIT: ");
Serial.print("  LOL_A: ");
Serial.print("  LOL_B: ");
Serial.print("  LOS: ");
Serial.print("  REVID: ");

Setting the Output Frequency

As indicated above, the library accepts and indicates clock and PLL frequencies in units of 0.01 Hz, as an unsigned long long variable type (or _uint64t). When entering literal values, append "ULL" to make an explicit unsigned long long number to ensure proper tuning. Since many applications won't require sub-Hertz tuning, you may wish to use an unsigned long (or _uint32t) variable to hold your tune frequency, then scale it up by multiplying by 100ULL before passing it to the set_freq() method.

The most simple way to set the output frequency is to let the library pick a PLL assignment for you. You do this by passing a 0 to the set_freq() method in the 2nd parameter:

si5351.set_freq(1014000000ULL, 0ULL, SI5351_CLK0);

If you let the library make PLL assignments, it will assign CLK0 to PLLA and both CLK1 and CLK2 to PLLB.

If that is not suitable, such as when you need glitch-free tuning or you are counting on multiple clocks being locked to the same reference, you may set the PLL frequency manually then make clock reference assignments to either of the PLLs.

Manually Selecting a PLL Frequency

The Si5351 consists of two main stages: two PLLs which are locked to the 25 or 27 MHz reference oscillator and which can be set from 600 to 900 MHz, and the output (multisynth) clocks which are locked to a PLL and can be set from 1 to 160 MHz. Instead of letting the library choose a PLL frequency for your chosen output frequency, you can choose it yourself by using the set_pll() method, and then using the same frequency in the second parameter in the call to set_freq() (as is shown in the example above).

Keep in mind when you are setting the PLL manually, that you need to be mindful of the limits of the IC. The multisynth is a fractional PLL, with limits described in AN619 as:

Valid Multisynth divider ratios are 4, 6, 8, and any fractional value between 8 + 1/1,048,575 and 900 + 0/1. This means that if any output is greater than 112.5 MHz (900 MHz/8), then this output frequency sets one of the VCO frequencies.

To put this in other words, if you want to manually set the PLL and wish to have an output frequency greater than 112.5 MHz, then the choice of PLL frequency is dictated by the choice of output frequency, and will need to be an even multiple of 4, 6, or 8.

Further Details

If we like we can adjust the output drive power:

si5351.drive_strength(SI5351_CLK0, SI5351_DRIVE_4MA);

The drive strength is the amount of current into a 50Ω load. 2 mA roughly corresponds to 7 dBm output and 8 mA is approximately 14 dBm output.

Individual outputs can be turned on and off. In the second argument, use a 0 to disable and 1 to enable:

si5351.output_enable(SI5351_CLK0, 0);

You may invert a clock output signal by using this command:

set_clock_invert(SI5351_CLK0, 1);


There will be some inherent error in the reference oscillator's actual frequency, so we can account for this by measuring the difference between the uncalibrated actual and nominal output frequencies, then using that difference as a correction factor in the library. The set_correction() method uses a signed integer calibration constant measured in parts-per-billion. The easist way to determine this correction factor is to measure a 10 MHz signal from one of the clock outputs (in Hz, or better resolution if you can measure it), scale it to parts-per-billion, then use it in the set_correction() method in future use of this particular reference oscillator. Once this correction factor is determined, it should not need to be measured again for the same reference oscillator/Si5351 pair unless you want to redo the calibration. With an accurate measurement at one frequency, this calibration should be good across the entire tuning range.

The calibration method is called like this:


A handy calibration program is provided with the library in the example folder named si5351calibration. To use it, simply hook up your Arduino to your Si5351, then connect it to a PC with the Arduino IDE. Connect the CLK0 output of the Si5351 to a frequency counter capable of measuring at 10 MHz (the more resolution, the better). Load the sketch then open the serial terminal window. Follow the prompts in the serial terminal to change the output frequency until your frequency counter reads exactly 10.000 000 00 MHz. The output from the Arduino on your serial terminal will tell you the correction factor you will need for future use of that reference oscillator/Si5351 combination.

One thing to note: the library is set for a 25 MHz reference crystal. If you are using a 27 MHz crystal, use the second parameter in the init() function to specify that as the reference oscillator frequency.


The phase of the output clock signal can be changed by using the set_phase() method. Phase is in relation to (and measured against the period of) the PLL that the output multisynth is referencing. When you change the phase register from its default of 0, you will need to keep a few considerations in mind.

Setting the phase of a clock requires that you manually set the PLL and take the PLL frequency into account when calculation the value to place in the phase register. As shown on page 10 of Silicon Labs Application Note 619 (AN619), the phase register is a 7-bit register, where a bit represents a phase difference of 1/4 the PLL period. Therefore, the best way to get an accurate phase setting is to make the PLL an even multiple of the clock frequency, depending on what phase you need.

If you need a 90 degree phase shift (as in many RF applications), then it is quite easy to determine your parameters. Pick a PLL frequency that is an even multiple of your clock frequency (remember that the PLL needs to be in the range of 600 to 900 MHz). Then to set a 90 degree phase shift, you simply enter that multiple into the phase register. Remember when setting multiple outputs to be phase-related to each other, they each need to be referenced to the same PLL.

You can see this in action in a sketch in the examples folder called si5351phase. It shows how one would set up an I/Q pair of signals at 14.1 MHz.

  // We will output 14.1 MHz on CLK0 and CLK1.
  // A PLLA frequency of 705 MHz was chosen to give an even
  // divisor by 14.1 MHz.
  unsigned long long freq = 1410000000ULL;
  unsigned long long pll_freq = 70500000000ULL;

  // Set PLLA to the chosen frequency
  si5351.set_pll(pll_freq, SI5351_PLLA);

  // Set CLK0 and CLK1 to use PLLA as the MS source
  si5351.set_ms_source(SI5351_CLK0, SI5351_PLLA);
  si5351.set_ms_source(SI5351_CLK1, SI5351_PLLA);

  // Set CLK0 and CLK1 to output 14.1 MHz with a fixed PLL frequency
  si5351.set_freq(freq, pll_freq, SI5351_CLK0);
  si5351.set_freq(freq, pll_freq, SI5351_CLK1);

  // Now we can set CLK1 to have a 90 deg phase shift by entering
  // 50 in the CLK1 phase register, since the ratio of the PLL to
  // the clock frequency is 50.
  si5351.set_phase(SI5351_CLK0, 0);
  si5351.set_phase(SI5351_CLK1, 50);

  // We need to reset the PLL before they will be in phase alignment


  • Two multisynths cannot share a PLL with when both outputs are < 1.024 MHz or >= 112.5 MHz. The library will refuse to set another multisynth to a frequency in that range if another multisynth sharing the same PLL is already within that frequency range.

  • Setting phase will be limited in the extreme edges of the output tuning ranges. Because the phase register is 7-bits in size and is denominated in units representing 1/4 the PLL period, not all phases can be set for all output frequencies. For example, if you need a 90° phase shift, the lowest frequency you can set it at is 4.6875 MHz (600 MHz PLL/128).

Public Methods


 * init(uint8_t xtal_load_c, uint32_t ref_osc_freq)
 * Setup communications to the Si5351 and set the crystal
 * load capacitance.
 * xtal_load_c - Crystal load capacitance. Use the SI5351_CRYSTAL_LOAD_*PF
 * defines in the header file
 * ref_osc_freq - Crystal/reference oscillator frequency in 1 Hz increments.
 * Defaults to 25000000 if a 0 is used here.
void Si5351::init(uint8_t xtal_load_c, uint32_t ref_osc_freq)


 * set_freq(uint64_t freq, uint64_t pll_freq, enum si5351_clock output)
 * Sets the clock frequency of the specified CLK output
 * freq - Output frequency in Hz
 * pll_freq - Frequency of the PLL driving the Multisynth
 *   Use a 0 to have the function choose a PLL frequency
 * clk - Clock output
 *   (use the si5351_clock enum)
uint8_t Si5351::set_freq(uint64_t freq, uint64_t pll_freq, enum si5351_clock clk)


 * set_pll(uint64_t pll_freq, enum si5351_pll target_pll)
 * Set the specified PLL to a specific oscillation frequency
 * pll_freq - Desired PLL frequency
 * target_pll - Which PLL to set
 *     (use the si5351_pll enum)
void Si5351::set_pll(uint64_t pll_freq, enum si5351_pll target_pll)


 * set_ms(enum si5351_clock clk, struct Si5351RegSet ms_reg, uint8_t int_mode, uint8_t r_div, uint8_t div_by_4)
 * Set the specified multisynth parameters. Not normally needed, but public for advanced users.
 * clk - Clock output
 *   (use the si5351_clock enum)
 * int_mode - Set integer mode
 *  Set to 1 to enable, 0 to disable
 * r_div - Desired r_div ratio
 * div_by_4 - Set Divide By 4 mode
 *   Set to 1 to enable, 0 to disable
void Si5351::set_ms(enum si5351_clock clk, struct Si5351RegSet ms_reg, uint8_t int_mode, uint8_t r_div, uint8_t div_by_4)


 * output_enable(enum si5351_clock clk, uint8_t enable)
 * Enable or disable a chosen output
 * clk - Clock output
 *   (use the si5351_clock enum)
 * enable - Set to 1 to enable, 0 to disable
void Si5351::output_enable(enum si5351_clock clk, uint8_t enable)


 * drive_strength(enum si5351_clock clk, enum si5351_drive drive)
 * Sets the drive strength of the specified clock output
 * clk - Clock output
 *   (use the si5351_clock enum)
 * drive - Desired drive level
 *   (use the si5351_drive enum)
void Si5351::drive_strength(enum si5351_clock clk, enum si5351_drive drive)


 * update_status(void)
 * Call this to update the status structs, then access them
 * via the dev_status and dev_int_status global variables.
 * See the header file for the struct definitions. These
 * correspond to the flag names for registers 0 and 1 in
 * the Si5351 datasheet.
void Si5351::update_status(void)


 * set_correction(int32_t corr)
 * Use this to set the oscillator correction factor to
 * EEPROM. This value is a signed 32-bit integer of the
 * parts-per-billion value that the actual oscillation
 * frequency deviates from the specified frequency.
 * The frequency calibration is done as a one-time procedure.
 * Any desired test frequency within the normal range of the
 * Si5351 should be set, then the actual output frequency
 * should be measured as accurately as possible. The
 * difference between the measured and specified frequencies
 * should be calculated in Hertz, then multiplied by 10 in
 * order to get the parts-per-billion value.
 * Since the Si5351 itself has an intrinsic 0 PPM error, this
 * correction factor is good across the entire tuning range of
 * the Si5351. Once this calibration is done accurately, it
 * should not have to be done again for the same Si5351 and
 * crystal. The library will read the correction factor from
 * EEPROM during initialization for use by the tuning
 * algorithms.
void Si5351::set_correction(int32_t corr)


 * set_phase(enum si5351_clock clk, uint8_t phase)
 * clk - Clock output
 *   (use the si5351_clock enum)
 * phase - 7-bit phase word
 *   (in units of VCO/4 period)
 * Write the 7-bit phase register. This must be used
 * with a user-set PLL frequency so that the user can
 * calculate the proper tuning word based on the PLL period.
void Si5351::set_phase(enum si5351_clock clk, uint8_t phase)


 * get_correction(void)
 * Returns the oscillator correction factor stored
 * in RAM.
int32_t Si5351::get_correction(void)


 * pll_reset(enum si5351_pll target_pll)
 * target_pll - Which PLL to reset
 *     (use the si5351_pll enum)
 * Apply a reset to the indicated PLL.
void Si5351::pll_reset(enum si5351_pll target_pll)


 * set_ms_source(enum si5351_clock clk, enum si5351_pll pll)
 * clk - Clock output
 *   (use the si5351_clock enum)
 * pll - Which PLL to use as the source
 *     (use the si5351_pll enum)
 * Set the desired PLL source for a multisynth.
void Si5351::set_ms_source(enum si5351_clock clk, enum si5351_pll pll)


 * set_int(enum si5351_clock clk, uint8_t int_mode)
 * clk - Clock output
 *   (use the si5351_clock enum)
 * enable - Set to 1 to enable, 0 to disable
 * Set the indicated multisynth into integer mode.
void Si5351::set_int(enum si5351_clock clk, uint8_t enable)


 * set_clock_pwr(enum si5351_clock clk, uint8_t pwr)
 * clk - Clock output
 *   (use the si5351_clock enum)
 * pwr - Set to 1 to enable, 0 to disable
 * Enable or disable power to a clock output (a power
 * saving feature).
void Si5351::set_clock_pwr(enum si5351_clock clk, uint8_t pwr)


 * set_clock_invert(enum si5351_clock clk, uint8_t inv)
 * clk - Clock output
 *   (use the si5351_clock enum)
 * inv - Set to 1 to enable, 0 to disable
 * Enable to invert the clock output waveform.
void Si5351::set_clock_invert(enum si5351_clock clk, uint8_t inv)


 * set_clock_source(enum si5351_clock clk, enum si5351_clock_source src)
 * clk - Clock output
 *   (use the si5351_clock enum)
 * src - Which clock source to use for the multisynth
 *   (use the si5351_clock_source enum)
 * Set the clock source for a multisynth (based on the options
 * presented for Registers 16-23 in the Silicon Labs AN619 document).
 * Choices are XTAL, CLKIN, MS0, or the multisynth associated with
 * the clock output.
void Si5351::set_clock_source(enum si5351_clock clk, enum si5351_clock_source src)


 * set_clock_disable(enum si5351_clock clk, enum si5351_clock_disable dis_state)
 * clk - Clock output
 *   (use the si5351_clock enum)
 * dis_state - Desired state of the output upon disable
 *   (use the si5351_clock_disable enum)
 * Set the state of the clock output when it is disabled. Per page 27
 * of AN619 (Registers 24 and 25), there are four possible values: low,
 * high, high impedance, and never disabled.
void Si5351::set_clock_disable(enum si5351_clock clk, enum si5351_clock_disable dis_state)


 * set_clock_fanout(enum si5351_clock_fanout fanout, uint8_t enable)
 * fanout - Desired clock fanout
 *   (use the si5351_clock_fanout enum)
 * enable - Set to 1 to enable, 0 to disable
 * Use this function to enable or disable the clock fanout options
 * for individual clock outputs. If you intend to output the XO or
 * CLKIN on the clock outputs, enable this first.
 * By default, only the Multisynth fanout is enabled at startup.
void Si5351::set_clock_fanout(enum si5351_clock_fanout fanout, uint8_t enable)


 * set_pll_input(enum si5351_pll pll, enum si5351_pll_input input)
 * pll - Which PLL to use as the source
 *     (use the si5351_pll enum)
 * input - Which reference oscillator to use as PLL input
 *     (use the si5351_pll_input enum)
 * Set the desired reference oscillator source for the given PLL.
void Si5351::set_pll_input(enum si5351_pll pll, enum si5351_pll_input input)


uint8_t Si5351::si5351_write_bulk(uint8_t addr, uint8_t bytes, uint8_t *data)


uint8_t Si5351::si5351_write(uint8_t addr, uint8_t data)


uint8_t Si5351::si5351_read(uint8_t addr)


Here are the defines, structs, and enumerations you will find handy to use with the library.

Crystal load capacitance:


Clock outputs:

enum si5351_clock {SI5351_CLK0, SI5351_CLK1, SI5351_CLK2, SI5351_CLK3,
  SI5351_CLK4, SI5351_CLK5, SI5351_CLK6, SI5351_CLK7};

PLL sources:

enum si5351_pll {SI5351_PLLA, SI5351_PLLB};

Drive levels:

enum si5351_drive {SI5351_DRIVE_2MA, SI5351_DRIVE_4MA, SI5351_DRIVE_6MA, SI5351_DRIVE_8MA};

Clock sources:

enum si5351_clock_source {SI5351_CLK_SRC_XTAL, SI5351_CLK_SRC_CLKIN, SI5351_CLK_SRC_MS0, SI5351_CLK_SRC_MS};

Clock disable states:

enum si5351_clock_disable {SI5351_CLK_DISABLE_LOW, SI5351_CLK_DISABLE_HIGH, SI5351_CLK_DISABLE_HI_Z, SI5351_CLK_DISABLE_NEVER};

Clock fanout:

enum si5351_clock_fanout {SI5351_FANOUT_CLKIN, SI5351_FANOUT_XO, SI5351_FANOUT_MS};

PLL input sources:

enum si5351_pll_input{SI5351_PLL_INPUT_XO, SI5351_PLL_INPUT_CLKIN};

Status register:

struct Si5351Status
  uint8_t SYS_INIT;
  uint8_t LOL_B;
  uint8_t LOL_A;
  uint8_t LOS;
  uint8_t REVID;

Interrupt register:

struct Si5351IntStatus
  uint8_t SYS_INIT_STKY;
  uint8_t LOL_B_STKY;
  uint8_t LOL_A_STKY;
  uint8_t LOS_STKY;

Raw Commands

If you need to read and write raw data to the Si5351, there is public access to the library's read(), write(), and write_bulk() methods.

Changes from alpha version

  • Added reference oscillator frequency parameter to init()
  • Changed the correction factor from parts-per-ten million to parts-per-billion


Right now, this code is focused solely on the 3-output 10-MSOP variant (Si5351A3). Since some of the code was derived from the Si5351 driver in the Linux kernel, it may be useable on with the other variants, but certainly many features won't work yet. With any luck, we will get the library to work with the other variants as well, or even better, maybe someone will take the initiative, write the code, and send me a pull request.


  • v1.1.2

    Fix error where register 183 is not pre-loaded with correct value per AN619. Add define for SI5351_CRYSTAL_LOAD_0PF (undocumented in AN619 but present in the official ClockBuilder software).

  • v1.1.1

    Fix if statement eval error in set_clock_disable()

  • v1.1.0

    Added set_pll_input() method to allow toggling the PLL reference source for the Si5351C variant and added support to init() for different PLL reference frequencies from 10 to 100 MHz.

  • v1.0.0

    Initial release


  • Create an interface to the ref osc frequency
  • Implement clock disable state
  • Implement invert
  • Implement power up/down
  • Implement phase register access
  • Implement tuning above 150 MHz
  • Implement tuning below 1 MHz
  • Implement sub-Hz tuning
  • Implement PLL reset