An ultra-low frequency ring oscillator with programmable tracking using a phase-locked loop

This paper presents a programmable ultra-low frequency ring oscillator designed in a 0.5μm CMOS technology with 3.3 V power supply. In normal operation, the oscillator produces a periodic signal without external inputs (other than power and ground). During programming, the oscillator tracks and memorizes the input reference frequency using a phase-locked loop (PLL) architecture. The PLL controls an on-chip nonvolatile memory using a floating gate structure; the voltage on the floating gate node is applied to the voltage-controlled oscillator (VCO) to generate the desired frequency. A high-voltage NMOS is introduced to realize the floating gate control; measurement results show that the breakdown voltage is close to 40 V. Transient simulations indicate that the circuit successfully tracks a desired frequency with maximum period jitter of 0.49 μs at the VCO output which translates to frequency error of 0.88 % at the system output.