  We present the design and characterization of the first full chain precision
timing prototype ASIC, named ETL Readout Chip version 1 (ETROC1) for the CMS
MTD endcap timing layer (ETL) upgrade. The ETL utilizes Low Gain Avalanche
Diode (LGAD) sensors to detect charged particles, with the goal to achieve a
time resolution of 40 - 50 ps per hit, and 30 - 40 ps per track with hits from
two detector layers. The ETROC1 is composed of a 5 x 5 pixel array and
peripheral circuits. The pixel array includes a 4 x 4 active pixel array with
an H-tree shaped network delivering clock and charge injection signals. Each
active pixel is composed of various components, including a bump pad, a charge
injection circuit, a pre-amplifier, a discriminator, a digital-to-analog
converter, and a time-to-digital converter. These components play essential
roles as the front-end link in processing LGAD signals and measuring
timing-related information. The peripheral circuits provide clock signals and
readout functionalities. The size of the ETROC1 chip is 7 mm x 9 mm. ETROC1 has
been fabricated in a 65 nm CMOS process, and extensively tested under stimuli
of charge injection, infrared laser, and proton beam. The time resolution of
bump-bonded ETROC1 + LGAD chipsets reaches 42 - 46 ps per hit in the beam test.
