This table gives cross-reference parts and alternative options found for AT89C51RE2-SLSUM. The Form Fit Function (FFF) tab will give you the options that are more likely to serve as direct pin-to-pin alternates or drop-in parts. The Functional Equivalents tab will give you options that are likely to match the same function of AT89C51RE2-SLSUM, but it may not fit your design. Always verify details of parts you are evaluating, as these parts are offered as suggestions for what you are looking for and are not guaranteed.