def pwm_start(self, channel, duty_cycle=None, frequency=None):
        """
        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``

        """
        if frequency:
            self.set_pwm_freq(frequency)
        self.set_pwm(channel, 0, int(4096 * (duty_cycle/100)))
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``