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8151GS–AVR–07/10

ATmega128A

The ATmega128A is 100% pin compatible with ATmega103, and can replace the ATmega103

on current Printed Circuit Boards. The application note “Replacing ATmega103 by

ATmega128A” describes what the user should be aware of replacing the ATmega103 by an

ATmega128A.

2.2.1

ATmega103 Compatibility Mode

By programming the M103C fuse, the ATmega128 will be compatible with the ATmega103

regards to RAM, I/O pins and interrupt vectors as described above. However, some new fea-

tures 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 sections.

• In addition, there are some other minor differences to make it more compatible to

ATmega103:

• 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.

Unused I/O bits in ATmega103 should be written to 0 to ensure same operation in ATmega128.

2.3

Pin Descriptions

2.3.1

VCC

Digital supply voltage.

2.3.2

GND

Ground.

2.3.3

Port A (PA7:PA0)

Port A is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The

Port A output buffers have symmetrical drive characteristics with both high sink and source

capability. As inputs, Port A pins that are externally pulled low will source current if the pull-up

resistors are activated. The Port A pins are tri-stated when a reset condition becomes active,

even if the clock is not running.

Port A also serves the functions of various special features of the ATmega128A as listed on