  The increase in luminosity, and consequent higher backgrounds, of the LHC
upgrades require improved rejection of fake tracks in the forward region of the
ATLAS Muon Spectrometer. The New Small Wheel upgrade of the Muon Spectrometer
aims to reduce the large background of fake triggers from track segments that
are not originated from the interaction point. The New Small Wheel employs two
detector technologies, the resistive strip Micromegas detectors and the "small"
Thin Gap Chambers, with a total of 2.45 Million electrodes to be sensed. The
two technologies require the design of a complex electronics system given that
it consists of two different detector technologies and is required to provide
both precision readout and a fast trigger. It will operate in a high background
radiation region up to about 20 kHz/cm$^{2}$ at the expected HL-LHC luminosity
of $\mathcal{L}$=7.5$\times10^{34}$cm$^{-2}$s$^{-1}$. The architecture of the
system is strongly defined by the GBTx data aggregation ASIC, the
newly-introduced FELIX data router and the software based data handler of the
ATLAS detector. The electronics complex of this new detector was designed and
developed in the last ten years and consists of multiple radiation tolerant
Application Specific Integrated Circuits, multiple front-end boards, dense
boards with FPGA's and purpose-built Trigger Processor boards within the ATCA
standard. The New Small Wheel has been installed in 2021 and is undergoing
integration within ATLAS for LHC Run 3. It should operate through the end of
Run 4 (December 2032). In this manuscript, the overall design of the New Small
Wheel electronics is presented.
