some new features in ATmega128 are not available in this compatibility mode,
these features are listed below:
• One USART instead of two, Asynchronous mode only. Only the eight least
significant bits of the Baud Rate Register is available.
• One 16 bits Timer/Counter with two compare registers instead of two 16-bit
Timer/Counters with three compare registers.
• Two-wire serial interface is not supported.
• Port C is output only.
• Port G serves alternate functions only (not a general I/O port).
• Port F serves as digital input only in addition to analog input to the ADC.
• Boot Loader capabilities is not supported.
• It is not possible to adjust the frequency of the internal calibrated RC Oscillator.
• The External Memory Interface can not release any Address pins for general I/O,
neither configure different wait-states to different External Memory Address
In addition, there are some other minor differences to make it more compatible to
• Only EXTRF and PORF exists in MCUCSR.
• Timed sequence not required for Watchdog Time-out change.
• External Interrupt pins 3 - 0 serve as level interrupt only.
• USART has no FIFO buffer, so data overrun comes earlier.