Patent ID: 7502061

Claim:
A method for operating an image sensor comprising an array of digital pixels, and a memory connected to the array of digital pixels, the memory comprising first and second frame stores, the method comprising: applying a reference voltage to pixels of the array of digital pixels; obtaining reference data from the array of digital pixels by performing a reset phase for setting a voltage of a digital pixel to a value by applying a reset signal thereto, the reset signal being deasserted at an end of the reset phase, performing a calibration phase after the reset phase, the reference voltage being varied through the calibration phrase, measuring an offset time which is the time taken from a start of the calibration phase to when the reference voltage and a voltage provided at the digital pixel differs by a threshold amount greater than zero, and using the offset time as an indication of the pixel's offset; storing the reference data in the first frame store; obtaining image data from the array of digital pixels; storing the image data in the second frame store; and comparing the reference data and image data to obtain an image in which the offsets have been cancelled.