  We present the detailed study of the digital readout of Topmetal-II- CMOS
pixel direct charge sensor. Topmetal-II- is an integrated sensor with an array
of 72X72 pixels each capable of directly collecting external charge through
exposed metal electrodes in the topmost metal layer. In addition to the
time-shared multiplexing readout of the analog output from Charge Sensitive
Amplifiers in each pixel, hits are also generated through comparators with
individually DAC settable thresholds in each pixel. The hits are read out via a
column-based priority logic structure, retaining both hit location and time
information. The in-array column-based priority logic is fully combinational
hence there is no clock distributed in the pixel array. Sequential logic and
clock are placed on the peripheral of the array. We studied the detailed
working behavior and performance of this readout, and demonstrated its
potential in imaging applications.
