A Digitally-Controlled Ring Oscillator in 28 nm CMOS technology

A digitally-controlled ring oscillator for phase locked loops designed in a commercial 28 nm CMOS technology is presented. Its operating frequency ranges from 2 GHz to 3.2 GHz. A much wider frequency range available in typical case compensates for frequency limitations induced by process variability. The circuit is based on a ring oscillator in which the switching speed of the inverters is controlled by a stream of digital bits. This oscillator is part of a digital PLL, in which the frequency of this oscillator is divided by 8 and used to track an incoming clock signal between 250 MHz and 400 MHz. This circuit is used to obtain eight different clock signals, synchronized with the reference one, to be distributed in massive parallel computation VLSI devices in order to spread the total power consumption of the chip across the reference clock cycle. Post-layout simulations demonstrate that the device is fully compliant in every process corner with the requirements of the future associative memory chip for the track trigger of the ATLAS detector at CERN.