A PWM DPS with pixel-level reconfigurable 4/8-bit counter/SRAM

A pulse width modulation (PWM) digital pixel sensor (DPS) is presented. The pixel includes a novel pixel-level reconfigurable 4/8-bit counter/SRAM, which can operate as a 4-bit counter or as an 8-bit memory. In contrast to the conventional PWM DPS in which all the encoding data are provided by a global counter, in our DPS, for the 8-bit digital data encoding the photodiode current, 4 bits are obtained from this pixel-level counter and the other 4 bits are provided by a global counter. The number of global data buses is thus reduced which in turn reduces the global bus capacitance and eases the routing. Moreover, the pixel can operate in 8-bit or in 4-bit conversion mode. 8-bit conversion provides a normal high resolution imaging while 4-bit conversion provides a higher frame rate. The pixel sensor has been designed and fabricated in a 0.35 μm CMOS technology and its operation is demonstrated through experimental results. Each pixel occupies an area of 46×48 μm² with a fill-factor of 14%.