to ground and at the other end to the other end of r2 and the drain of m20 the sources of m19 and m20 are connected to the gate of m3 precharge logic 430 is constructed of enhancement mode mosfet transistor m21 and resistors r4 and r5 the drain of m21 is connected to the gate of m4 selector 306 and capacitance c1 the source of m21 is connected to one end of r4 and r5 the other end of r4 is connected to a constant voltage and the other end of r5 is connected to ground finally the gate of m21 is connected to precharge signal vprech each key as mentioned operates as a variable capacitor in that the key capacitance varies with key depression since the key capacitance affects the voltage seen by sense amp 307 from the selected sense line comparison of the returned voltage with the voltage corresponding to the state of the key from the prior scan allows key position to be detected when vprech is asserted by sequencer 316 m21 is turned on and the divided voltage from r4 and r5 is allowed to pass to c1 and to the key after a length of time dependent on element values c1 and the key will have been charged to the level of the voltage passed by m21 from r4 and r5 this bias voltage will remain.