ATmega128
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:
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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.
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.
In addition, there are some other minor differences to make it more compatible to ATmega103:
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Unused I/O bits in ATmega103 should be written to 0 to ensure same operation in ATmega128.
Pin Descriptions
VCC
GND
Port A (PA7..PA0)
Digital supply voltage.
Ground.
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 ATmega128 as listed on
Port B (PB7..PB0)
Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port B output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up
resistors are activated. The Port B pins are tri-stated when a reset condition becomes active,
even if the clock is not running.
Port B also serves the functions of various special features of the ATmega128 as listed on
Port C (PC7..PC0)
Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The
Port C output buffers have symmetrical drive characteristics with both high sink and source
capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up
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2467S–AVR–07/09
ATMEL [ ATMEL CORPORATION ]
