code: def pwm_start(self, channel, duty_cycle=None, frequency=None):
          
          if frequency:
              self.set_pwm_freq(frequency)
          self.set_pwm(channel, 0, int(4096 * (duty_cycle/100)))
docs: Starts the pwm signal on a channel. The channel should be defined as pwm prior to this call.
          If no duty_cycle or frequency is passed in this call previous values from call to
          define_as_pwm or pwm_start is used.
  
          :param channel:
              The channel to start the pwm signal on.
  
          :type channel: ``int``
  
          :param duty_cycle:
              The duty cycle use on the channel.
  
          :type duty_cycle: ``int``
  
          :param frequency:
              The frequency to be used on the pwm channel.
  
          :type frequency: ``int``