Silicon Resonator Based 3.2 mu W Real Time Clock With +/- 10 ppm Frequency Accuracy

This paper presents an ultra-low power generic compensation scheme that is used to implement a real time clock based on an AlN-driven 1 MHz uncompensated silicon resonator achieving 3.2 mu W power dissipation at 1 V and +/- 10 ppm frequency accuracy over a 0-50 degrees C temperature range. It relies on the combination of fractional division and frequency interpolation for coarse and fine tuning respectively. By proper calibration and application of temperature dependent corrections, any frequency below that of the uncompensated resonator can be generated yielding programmability, resonator fabrication tolerances and temperature drift compensation without requiring a PLL. To minimize the IC area, a dual oscillator temperature measurement concept based on a ring oscillator/resistor thermal sensor was implemented yielding a resolution of 0.04 degrees C. The IC was fabricated on a 0.18 mu m 1P6M CMOS technology.