Reduced cost pointing stick circuit

Abstract

A low-cost circuit processes, with high precision, the output signals of a pointing stick used for controlling a cursor on a computer display screen. A two-stage method of high precision moderate range analog-to-digital (A/D) conversion is combined with a known method of A/D conversion by sweeping a reference voltage across the range of the voltage to be measured at a known rate and noting the time of coincidence. Operational amplifiers of moderate quality are used in an open-loop mode, so that the operational amplifier inputs are high impedance. In order to get sufficient isolation from power supply noise and variations, the analog circuitry is isolated from the supply voltage while measurements are being made. To avoid interference from the digital switching transients of the microprocessor with the analog comparator switching time, the microprocessor is placed in idle mode, thereby removing its internal clock drive signals (and associated transients) from all of the microprocessor's internal circuitry except the timer which is being used to make the measurement. The resulting circuitry is capable of detecting a very small signal in noise spikes many times larger.