The third run (Run 3) of the ATLAS experiment at CERN's Large Hadron Collider will begin in 2021 in a high luminosity and high energy environment. In this circumstance, an effective trigger system is required. ATLAS uses a two-stage trigger system. The first stage trigger system (Level 1 trigger) is based on hardware, and the second stage is a software-based high-level trigger on a PC farm. With this system, an overall trigger rate is required to be suppressed to 1 kHz or less from a bunch-crossing rate of 40 MHz. In the liquid argon calorimeter level 1 trigger system, new trigger readouts with a factor of ten higher granularity compared to the Run 2 trigger readout, are introduced to suppress backgrounds. By measuring shower shape with the higher granularity, the electron and photon trigger rate can be suppressed sufficiently. To process all data from the supercells, new readout boards are prepared. One of the boards, dubbed LATOME, converts ADC data to transverse energy with fixed latency and sends three types of energies to the Level 1 trigger system. The trigger system uses computed energies with LATOME firmware, and therefore the development and validation of the LATOME firmware is important for the ATLAS data acquisition system. One of the LATOME firmware module, User Code, calculates the transverse energy for all supercells. To ensure all User Code blocks are functional, firmware validation is required. Simulation based firmware verifications and validations with the LATOME are presented with the firmware design.