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LiteX Clock Control Driver Sample


This sample is providing an overview of LiteX clock control driver capabilities. The driver uses Mixed Mode Clock Manager (MMCM) module to generate up to 7 clocks with defined phase, frequency and duty cycle.


  • LiteX-capable FPGA platform with MMCM modules (for example Digilent Arty A7 development board)

  • SoC configuration with VexRiscv soft CPU and Xilinx 7-series MMCM interface in LiteX (S7MMCM module)

  • Optional: clock output signals redirected to output pins for testing


Basic configuration of the driver, including default settings for clock outputs, is held in Device Tree clock control nodes.

			clk0: clock-controller@0 {
				#clock-cells = <1>;
				reg = <0>;
				compatible = "litex,clkout";
				clock-output-names = "CLK_0";
				litex,clock-frequency = <100000000>;
				litex,clock-phase = <0>;
				litex,clock-duty-num = <1>;
				litex,clock-duty-den = <2>;
				litex,clock-margin = <1>;
				litex,clock-margin-exp = <2>;
				status = "disabled";
			clk1: clock-controller@1 {
				#clock-cells = <1>;
				reg = <1>;
				compatible = "litex,clkout";
				clock-output-names = "CLK_1";
				litex,clock-frequency = <100000000>;
				litex,clock-phase = <0>;
				litex,clock-duty-num = <1>;
				litex,clock-duty-den = <2>;
				litex,clock-margin = <1>;
				litex,clock-margin-exp = <2>;
				status = "disabled";
		clock0: clock@82005000 {
			compatible = "litex,clk";
			label = "clock0";
			reg = <0x82005000 0x1
			       0x82005004 0x1
			       0x82005008 0x1
			       0x8200500c 0x1
			       0x82005010 0x1
			       0x82005014 0x1
			       0x82005018 0x2
			       0x82005020 0x2>;
			reg-names = "drp_reset",
			#clock-cells = <1>;
			#address-cells = <1>;
			#size-cells = <1>;
			clocks = <&clk0 0>, <&clk1 1>;
			clock-output-names = "CLK_0", "CLK_1";
			litex,lock-timeout = <10>;
			litex,drdy-timeout = <10>;
			litex,sys-clock-frequency = <100000000>;
			litex,divclk-divide-min = <1>;
			litex,divclk-divide-max = <107>;
			litex,clkfbout-mult-min = <2>;
			litex,clkfbout-mult-max = <65>;
			litex,vco-freq-min = <600000000>;
			litex,vco-freq-max = <1200000000>;
			litex,clkout-divide-min = <1>;
			litex,clkout-divide-max = <126>;
			litex,vco-margin = <0>;
			status = "disabled";

This configuration defines 2 clock outputs: clk0 and clk1 with default frequency set to 100MHz, 0 degrees phase offset and 50% duty cycle. Special care should be taken when defining values for FPGA-specific configuration (parameters from litex,divclk-divide-min to litex,vco-margin).

Important note: reg properties in clk0 and clk1 nodes reference the clock output number (clkout_nr)

Driver Usage

The driver is interfaced with the Clock Control API function clock_control_on() and a LiteX driver specific structure:

struct litex_clk_setup

Structure for interfacing with clock control API.

  • clkout_nr: Number of clock output to be changed

  • rate: Frequency to set given in Hz

  • phase: Phase offset in degrees

  • duty: Duty cycle of clock signal in percent

To change clock parameter it is needed to cast a pointer to structure litex_clk_setup onto clock_control_subsys_t and use it with clock_control_on().
This code will try to set on clk0 frequency 50MHz, 90 degrees of phase offset and 75% duty cycle.
struct device *dev;
int ret;
struct litex_clk_setup setup = {
        .clkout_nr = 0,
        .rate = 50000000,
        .duty = 75,
        .phase = 90
dev = device_get_binding("clock0");
clock_control_subsys_t sub_system = (clock_control_subsys_t*)&setup;
if ((ret = clock_control_on(dev, sub_system)) != 0) {
        LOG_ERR("Set CLKOUT%d param error!", setup.clkout_nr);
        return ret;

Clock output status (frequency, duty and phase offset) can be acquired with function clock_control_get_status() and clock output frequency only can be queried with clock_control_get_rate()

In both getter functions, basic usage is similar to clock_control_on(). Structure litex_clk_setup is used to set clkout_nr of clock output from which data is to be acquired.

Sample usage

This example provides a simple way of checking various clock output settings. User can pick one of 4 possible scenarios:

  • Frequency range,

  • Duty cycle range,

  • Phase range,

  • Setting frequency, duty and phase at once, then check clock status and rate,

Scenarios are selected by defining LITEX_CLK_TEST as one of:





Code is performed on 2 clock outputs with clkout_nr defined in LITEX_CLK_TEST_CLK1 and LITEX_CLK_TEST_CLK2. Tests are controlled by separate defines for each scenario.


west build -b litex_vexriscv zephyr/samples/drivers/clock_control

Drivers prints a lot of useful debugging information to the log. It is highly recommended to enable logging and synchronous processing of log messages and set log level to Info.

Sample output

[00:00:00.200,000] <inf> CLK_CTRL_LITEX: CLKOUT0: set rate: 100000000 HZ
[00:00:00.240,000] <inf> CLK_CTRL_LITEX: CLKOUT1: updated rate: 100000000 to 100000000 HZ
[00:00:00.280,000] <inf> CLK_CTRL_LITEX: CLKOUT0: set duty: 50%
[00:00:00.320,000] <inf> CLK_CTRL_LITEX: CLKOUT0: set phase: 0 deg
[00:00:00.360,000] <inf> CLK_CTRL_LITEX: CLKOUT1: set rate: 100000000 HZ
[00:00:00.400,000] <inf> CLK_CTRL_LITEX: CLKOUT1: set duty: 50%
[00:00:00.440,000] <inf> CLK_CTRL_LITEX: CLKOUT1: set phase: 0 deg
[00:00:00.440,000] <inf> CLK_CTRL_LITEX: LiteX Clock Control driver initialized
*** Booting Zephyr OS build zephyr-v2.2.0-2810-g1ca5dda196c3  ***
Clock Control Example! riscv32
device name: clock0
clock control device is 0x40013460, name is clock0
Clock test
Single test
[00:00:00.510,000] <inf> CLK_CTRL_LITEX: CLKOUT0: set rate: 15000000 HZ
[00:00:00.550,000] <inf> CLK_CTRL_LITEX: CLKOUT0: set phase: 90 deg
[00:00:00.590,000] <inf> CLK_CTRL_LITEX: CLKOUT0: set duty: 25%
[00:00:00.630,000] <inf> CLK_CTRL_LITEX: CLKOUT1: set rate: 15000000 HZ
[00:00:00.670,000] <inf> CLK_CTRL_LITEX: CLKOUT1: set duty: 75%
Getters test
CLKOUT0: get_status: rate:15000000 phase:90 duty:25
CLKOUT0: get_rate:15000000
CLKOUT1: get_status: rate:15000000 phase:0 duty:75
CLKOUT1: get_rate:15000000
Clock test done returning: 0