ATLASpix3 is a 2×2 cm$^2$ high voltage CMOS sensor chip designed to meet the specifications of outer layers of ATLAS inner tracker. It is compatible with the hybrid pixel sensor ASIC RD53A in terms of electronic interface and geometry. ATLASpix3 is a depleted monolithic CMOS pixel detector which allows the construction of quad modules of the same size as that of hybrid sensors. The readout scheme can be externally configured as triggered or triggerless column drain readout. The hit information is transmitted through a 1.28 Gbit/s serial link. The interface is based on a single command input that is used for providing clock, trigger and configuration commands. This
contribution summarizes the detector architecture with focus on the design of its readout circuitry.In addition, simulation results obtained using ReadOut Modelling Environment (ROME), that led to the design of the readout system are discussed.