Upgrade of the ATLAS Thin Gap Chambers Electronics for HL-LHC

The High-Luminosity LHC (HL-LHC) is planned to start the operation in 2026 with an instantaneous luminosity of $7.5 \times 10^{34} \mathrm{cm^{-2}s^{-1}}$. To cope with the event rate higher than that of LHC, the trigger and readout electronics of ATLAS Thin Gap Chamber will need to be replaced. An advanced first-level trigger with fast tracking will be implemented with the transfer of all hit data from the frontend to the backend boards. Studies with the data taken by ATLAS indicate that the advanced trigger could reduce the event rate by 30% for a single muon trigger with a transverse momentum threshold of 15 GeV while maintaining similar efficiency. First prototype of the frontend board has been developed with full functions required for HL-LHC including the data transfer of 256 channels with a 16 Gbps bandwidth and the control of the discriminator threshold. The data transfer has been demonstrated with charged particle beam at the CERN SPS beam facility. The control of the discriminator threshold has also been demonstrated, and a perfect linearity between the set and the measured values was obtained. We will present the overall design of the new trigger and readout electronics as well as the demonstration of the frontend board prototype.