Changing the modulation frequency during the control execution may be useful in applications such as resonant converters or in case of dynamic reconfiguration (start-up of drives for instance). This note shows how the B-Board PRO and B-Box RCP support the real-time tuning of the modulators' switching frequency.

The variable-frequency feature has been introduced in version 3.4 for C++ and in version 3.6 for ACG (Simulink & PLECS).
The installer for the latest SDK is available on imperix website, under SupportDownloads (


As a CB-PWM block can be freely mapped to any of the four available CLK blocks, variable-frequency modulation is designed to be implemented by:

  • Mapping the CB-PWM block that must have a variable frequency to a specific CLK.

  • Changing the frequency of the corresponding CLK during the execution.


  • More than one variable-frequencies can be used simultaneously.

  • The interrupt and the sampling processes are linked by design to the same CLK (CLOCK_0) and their frequency can not be changed during the execution. Consequently, an additional CLK block, mapped on CLOCK_1, CLOCK_2, or CLOCK_3, must be used for variable frequency operation.

  • The CLK blocks are implemented such that frequency changes are glitch-less, i.e. does not generate any unexpected behavior during the transition. Any frequency step can therefore be done at any time during operation.

ACG example


C++ example

As shown in the following code snippet, the frequency change is performed by re-configuring the clock generator CLOCK_1 during real-time control execution. This example uses the following configuration:


Linked to



Interrupt and sampling

Fixed frequency (50kHz)



Variable frequency

tUserSafe UserInit(void)
  // The interrupt and sampling use CLOCK_0
  Clock_SetFrequency(CLOCK_0, 50e3);
  ConfigureMainInterrupt(UserInterrupt, CLOCK_0, 0.5);
  // The PWM_CHANNEL_0 uses CLOCK_1 which is set as real-time tunable	
  Clock_SetFrequency(CLOCK_1, 10e3);
  CbPwm_ConfigureClock(PWM_CHANNEL_0, CLOCK_1);
  CbPwm_ConfigureOutputMode(PWM_CHANNEL_0, COMPLEMENTARY);
  CbPwm_ConfigureCarrier(PWM_CHANNEL_0, TRIANGLE);
  CbPwm_ConfigureDeadTime(PWM_CHANNEL_0, 1e-6);
  CbPwm_SetDutyCycle(PWM_CHANNEL_0, 0.5);
  Adc_ConfigureInput(0, GAIN_I, 0.0);
  Adc_ConfigureInput(1, GAIN_V, 0.0);

  return SAFE;

// Global variables can be observed from BB Control
float current;
float voltage;
float freq;

tUserSafe UserInterrupt(void)
  current = Adc_GetValue(0);
  voltage = Adc_GetValue(1);
  freq = GetOptimizedFrequency(current, voltage);
  Clock_SetFrequency(CLOCK_0, freq);
  return SAFE;
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