  We present a readout chip prototype for future pixel detectors with timing
capabilities. The prototype is intended for characterizing 4D pixel arrays with
a pixel size of $100\times100~\mu \text{m}^2$, where the sensors are Low Gain
Avalanche Diodes (LGADs). The long-term focus is towards a possible replacement
of disks in the extended forward pixel system (TEPX) of the CMS experiment
during the High Luminosity LHC (HL-LHC). The requirements for this ASIC are the
incorporation of a Time to Digital Converter (TDC) within each pixel, low power
consumption, and radiation tolerance up to
$5\times10^{15}~n_\text{eq}\text{~cm}^{-2}$ to withstand the radiation levels
in the innermost detector modules for $3000 \text{fb}^{-1}$ of the HL-LHC (in
the TEPX). A prototype has been designed and produced in 110~nm CMOS technology
at LFoundry and UMC with different versions of TDC structures, together with a
front end circuitry to interface with the sensors. The design of the TDC will
be discussed, with the test set-up for the measurements, and the first results
comparing the performance of the different structures.
