This application note is a guide to assist current AT90S4433 users in converting existing designs to ATmega48. ATMEGA48 is not designed to be a replacement for AT90S4433 but is pin compatible and has a very similar feature set. ATMEGA48 contains more SRAM memory than AT90S4433 and it has additional peripheral modules and features. However with a few modifications to the I/O register access, ATMEGA48CAN replace AT90S4433 on existing circuit boards. In addition to the functional changes, the electrical characteristics of the ATMEGA48 are different, including an increase in operating frequency ranges. Check the datasheet for detailed information. Improvements and added features in ATMEGA48 that are not in conflict with those in AT90S4433 are not covered by the scope of this document. Throughout this document it is assumed that ATMEGA48 factory-default settings are used unless otherwise noted.