32 c as indicated above the control assembly operates to open a valve in the frac sub assembly to enable fracturing treatment of a zone of interest above a hydraulic threshold pressure 0067 1 open the frac ports 38 as hydraulic pressure rises above a threshold value 0068 2 keep the frac ports 38 open when the hydraulic pressure drops below the threshold value until a lower threshold pressure is reached and 0069 3 close the frac ports 38 when the hydraulic pressure drops below the lower threshold pressure to accomplish these functions sub systems of springs pistons and hydraulic channels within the control assembly interact to channel hydraulic fluid to different sub systems depending on the uphole hydraulic pressure these sub systems include inter aila a high pressure piston 42 a low pressure piston 46 a return spring 29 a switch return spring 14 and associated hydraulic channels and chambers as will be described in greater detail below with reference to fig2 4 5 7 and 8 an overview of the operation of the sub systems is described with respect to changes in the uphole hydraulic pressure shown as threshold pressures a b ace c in fig8 at a hydraulic pressure below a fluid is circulated between through the circulation nozzles and the frac ports are closed at hydraulic pressure a the upper and lower packer elements are set at hydraulic pressure b the hydraulic pressure acting on high pressure piston 42 overcomes the switch return spring which causes the high pressure piston 42 and pressure switch assembly 15.