The LM3S5C51-IQC80-A1T is a Texas Instruments Stellaris® ARM® Cortex®-M3 MCU from the 5000 series, built around a 32-bit single-core processor running at 80 MHz.
Obsolete — planning the BOM transition
The official record does not list a direct successor order code. Any new project should target a current-production Cortex-M3 or Cortex-M4 part with similar peripheral set and package footprint.
