tPix is the front-end ASIC for Tixel detector aimed for momentum spectroscopy experiments at LCLS. It has 100mx100m pixels arranged in a 176x192 matrix (48x48 prototype) bump-bonded to a dedicated thin silicon sensor. The ASIC electronics measure the charge from the sensor and determine as well as digitize the timing information for each pixel of the detector with 100ps resolution. The timing information is extracted from the sensor signal with a charge-sensitive amplifier (CSA) followed by a discriminator. The time-to-digital conversion is performed in two steps: an 8 bit global counter performs the first coarse measurement while an in-pixel delay-line performs a 6/10 bit high-resolution fine measurement of the time-interval. The 14/18 bit conversion result is read out in between pulse intervals during the readout phase. The chip will be able to sustain a frame rate up to 1KHz. A 48x48 pixel-array prototype has been fabricated in TSMC 0.13m CMOS technology and its design and initial characterization are reported here. Show Partial Abstract