GCBASIC documentation
Pulse width modulation

Introduction:

The methods described in this section allow the generation of Pulse Width Modulation (PWM) signals. PWM signals enables the microcontroller to control items like the speed of a motor, or the brightness of a LED or lamp.

The methods can also be used to generate the appropriate frequency signal to drive an infrared LED for remote control applications.

GCBASIC support the four different method shown below:
    - Two methods use the microcontroller CCP module
    - One method uses the microcontroller PWM module, and
    - One method is a software emulation of PWM.

Hardware PWM using a CCP module

Using PWM with the CCP module: This option requires a CCP module within the microcontroller.

Hardware PWM is only available through the "CCP" or "CCPx" pin. This is a hardware limitation of Microchip PIC microcontrollers.

Microcontrollers with PPS can change the pin - use the PPS tool to set the desired output pin.

This method uses three parameters to setup the PWM.

    'HPWM channel, frequency, duty cycle
    HPWM 1, 76, 80          ' <<< the HPWM instruction

Key line: HPWM 1, 76, 80 — drives CCP channel 1 at a 76 kHz carrier frequency with an 80% duty cycle; unlike the software PWM method below, this runs continuously in the background using dedicated hardware, freeing the program to do other work.

Hardware PWM using a PWM module

Using microcontroller PWM module. This option requires a PWM module within the microcontroller Microcontrollers with PPS can change the pin - use the PPS tool to set the desired output pin.

This method uses four parameters to setup the PWM.

    'HPWM channel, frequency, duty cycle, timer
    HPWM 5, 76, 80, 2

Hardware PWM using the CCP1 in fixed mode

Using Hardware PWM on fixed mode PWM requires a CCP1 module.

The fixed mode can use CCP1 only, and, the parameters of the PWM cannot be dynamically changed in the user program.   The parameters are fixed by the definition of two constants.

    #define PWM_Freq 76    'Set frequency in KHz
    #define PWM_Duty 80    'Set duty cycle to 80 %

    HPWMOn

    wait 5 s

    HPWMOff

Software PWM

Using Software PWM on requires no specific modules with the microcontroller.

The PWM parameters for duty and the number of pulses can be changed dynamically in the user program.

The PWM is only operational for the number of cycles stated in the calling method.

    'A call to use the software PWM on the specific port, with a duty of 127 for 100 cycles

    ; ----- Constants
    'PWM constant. This is a required constant.
    #define PWM_Out1 portb.0

  ; ----- Define Hardware settings
    'PWM port out.  This is not required but a good practice.
    dir PWM_Out1 out

    'Pulse the PWM
    PWMOut 1, 127, 100          ' <<< the PWMOut instruction

Key line: PWMOut 1, 127, 100 — generates 100 pulses at roughly 50% duty (127 of 255) purely in software, on any output pin; because no dedicated hardware is used, this call blocks the program until all 100 cycles finish, unlike the hardware methods above which run independently once started.

Relevant Constants:

A number of constants are used to control settings for the PWM hardware module of the microcontroller. To set them, place a line in the main program file that uses #define to assign a value to the particular constant.

See Also: