A timing circuit operates by applying an arbitrary voltage across a resistance (Rl) , and using the resulting current to generate a charging current which charges and/or discharges a capacitance (Cl) to an endpoint voltage. Additional circuitry is arranged such that the capacitance (Cl) is charged and/or discharged until its voltage crosses a threshold which is proportional to one of the resistance's endpoint voltages, such that the capacitance's endpoint voltage tracks the resistance's endpoint voltage. Thus, the resistor voltage can vary with supply voltage or temperature, or the resistance value itself can vary, without materially affecting the timing relationships. The arbitrary voltage is preferably provided with a pair of diode-connected transistors (MPl, MNl) connected in series with the resistance (Rl) , so that a single transistor operated at the same current density as one of the diode-connected transistors establishes the threshold voltage and detects when the capacitor voltage reaches the threshold.