This paper reports an embedded crystal (eCrystal) oscillator which is capable of compensating process, voltage, and temperature (PVT) variation. The delay behaviors of two different delay cells are exploited for the delay estimation. Then the estimated delay is parameterized by a second order approximated function. According to the estimated delay, the eCrystal oscillator is able to calibrate the frequency error without any external reference. A 40MHz oscillator is implemented in a 90-nm CMOS technology with area of 0.4mm
by an all-digital approach. The fabricated chip is measured under supply voltage from 0.9V to 1.1V and temperature range from 0°C to 75°C with active power consumption of 237μW.