PWMOut

Syntax:

    PWMOut channel, duty cycle, cycles

Command Availability:

Available on all microcontrollers. This method does NOT require a PWM module within the microcontroller.

This command uses a software PWM routine within GCBASIC to produce a PWM signal on the selected port of the chip.

The method PWMOut does not make use of any special hardware within the microcontroller. The PWM signal is generated only while the PWMOut command is executing - therefore, when the PWMOut is not executing by moving onto the next command, the PWM signal will stop.

Explanation :

channel sets the channel that the PWM is to be generated on. This must have been defined previously by setting the constants PWM_OUT1

PWM_OUT2, PWM_OUT3 or PWM_OUT4. The maximum number of channels available is 4.

duty cycle specifies the PWM duty cycle, and ranges from 0 to 255. 255 corresponds to 100%, 127 to 50%, 63 to 25%, and so on.

cycles is used to set the amount of PWM pulses to supply. This is useful for situations in which a pulse of a specific length is required.

The formula for calculating the time taken for one cycle is:

    TCYCLE = (28 + 10C)TOSC+ (255 * PWM_DELAY)

where:

   -C is the number of channels used
   -TOSC is the length of time taken to execute 1 instruction on the chip (0.2 us on a 20 MHz chip, 1 us on a 4 Mhz chip)
   -PWM_DELAY is a length of time specified using the PWM_DELAY constant

Example 1 :

    'This program controls the brightness of an LED on PORTB.0
    'using the software PWM routine and a potentiometer.
    #chip 16f877a, 20

    ; ----- Constants
    'PWM constant. This is a required constant.
    #define PWM_OUT1 portb.0

    ; ---- Optional Constant to add an delay after PWM pulse
    ''#Define PWM_DELAY 1 us


    ; ----- Define Hardware settings
    'PWM port out.  This is not required but good practice.
    dir PWM_OUT1 out

    'A potentiometer is attached to AN0

    ; ----- Variables
    ' No Variables specified in this example.

    ; ----- Main body of program commences here.
      do
          '100 cycles is a purely arbitrary value as the loop will maintain a relatively constant PWM
          PWMOut 1, ReadAD(AN0), 100          ' <<< the PWMOut instruction
      loop

end

Key line: PWMOut 1, ReadAD(AN0), 100 — drives channel 1 with a duty cycle read live from the potentiometer on AN0, repeating for 100 cycles per call; because the loop calls PWMOut again immediately, the pulse train continues without a visible gap.

Example 2 :

    'This program controls the brightness of an LED on gpio.1
    'using the software PWM routine and a potentiometer.
    #chip 12f675, 4

    ; ----- Constants
      'PWM constant. This is a required constant.
      #define PWM_OUT1 gpio.1

    ; ----- Define Hardware settings
      'PWM port out.  This is not required but good practice.
      dir PWM_OUT1 out

      'A potentiometer is attached to AN0

    ; ----- Variables
      ' No Variables specified in this example.

    ; ----- Main body of program commences here.
        do
          '100 cycles is a purely arbitrary value
            PWMOut 1, ReadAD(AN0), 100          ' <<< the PWMOut instruction, on a smaller 8-pin part
        loop
    end

Key line: PWMOut 1, ReadAD(AN0), 100 — identical call to Example 1, but on the 8-pin 12F675 using gpio.1 instead of portb.0; the command itself does not change with the chip, only the pin assigned to PWM_OUT1.

See Also:

  • PWMOn — hardware PWM alternative that does not need to be re-triggered in a loop
  • HPWM CCP — higher-resolution hardware PWM using a CCP module
  • Dir — setting the pin direction before driving it with PWM
  • ReadAD — reading the potentiometer value used as the duty cycle in the example above
  • Wait — introducing a fixed delay, as contrasted with a PWM pulse train
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