The maximum operating frequency of 555 is only 500KHz. It is recommended that you use high-speed digital integrated circuits as Schmitt triggers. You can use 74AS14, 74F14, and 74AC14, which are all Schmitt trigger-specific chips, each with 6 built-in Schmitt triggers. Note: 7414 and 74LS14 cannot be used as their operating frequency cannot be reached.