A digitally programmable on-chip RC-oscillator in 0.25 μm CMOS logic process

The design of a process compensated RC on-chip oscillator with internally generated current and voltage references is presented. The oscillator provides a selectable clock (30 MHz to 36 MHz) with digital trimming to minimize process variations (±45% in resistance value and ±15% in capacitance value). After fine-tuning, the oscillator guarantees less than ±2% frequency variation from 0 to 80 degrees, when operating down to 2 V. A 5-bit programmable fringe capacitor array was custom designed and modeled to overcome the lack of reliable capacitance devices in logic processes. The module was implemented in a 0.25 μm CMOS logic process, occupies an area of 444×280 μm² (bandgap excluded) and requires no external reference or components.