A variable-frequency digital ring oscillator provides small and consistent frequency adjustments throughout a locked range. The ring oscillator of the invention is standard cell placeable and operates at the technology limits to provide small and precise delay changes that is inexpensive to implement. The digital variable-frequency ring oscillator includes multiple macro delay elements forming an inverter ring circuit, each element having an individual macro delay unit that in turn is comprised of multiple adjustable delay units. All of these adjustable delay units are controlled by a single delay control signal.

An internal voltage generating circuit includes a first detector that compares an internal voltage and a first reference voltage to output a first detection signal. A second detector compares a supply voltage and a second reference voltage to output a second detection signal. A loop selection oscillator performs an oscillation operation in response to the first detection signal, selects a first loop or a second loop for performing the oscillation operation in response to the second detection signal, and outputs an oscillation signal. A charge pump performs a pumping operation according to the output of the loop selection oscillator and generates the internal voltage.