code: function() {
  		this.pulse1.doSweep();
  		this.pulse1.doLengthCounter();
  
  		this.pulse2.doSweep();
  		this.pulse2.doLengthCounter();
  
  		this.triangle.doLengthCounter();
  
  		this.noise.doLengthCounter();
  		// TODO
  	}
docs: Do half frame tick (sweeps and length counters).