The present invention is a method for specifying a constant in a digital electronic system. This technique employs the ratio of resistances between a reference resistor and a constant specifying resistor. The time constant of the constant specifying resistor and a capacitor is measured by discharging the capacitor, charging the capacitor through the constant specifying resistor and counting the number of predetermined time intervals required for the voltage across the capacitor to reach a predetermined threshold voltage. A similar time constant measurement is made for the reference resistor and the capacitor. The same conditions are employed as far as possible during these two measurements to insure the only differences are in the value of the resistances. The externally specified constant is computed from the digital ratio of the time constant of the constant specifying resistor and the time constant of the reference resistor. This technique is particularly adapted for use in a microprocessor system having several tristate output lines.