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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)))