The AVR core combines a rich instruction set with 32 general purpose working registers. All the
32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent
registers to be accessed in one single instruction executed in one clock cycle. The resulting
architecture is more code efficient while achieving throughputs up to ten times faster than con-
ventional CISC microcontrollers.
The ATmega128 provides the following features: 128K bytes of In-System Programmable Flash
with Read-While-Write capabilities, 4K bytes EEPROM, 4K bytes SRAM, 53 general purpose I/O
lines, 32 general purpose working registers, Real Time Counter (RTC), four flexible
Timer/Counters with compare modes and PWM, 2 USARTs, a byte oriented Two-wire Serial
Interface, an 8-channel, 10-bit ADC with optional differential input stage with programmable
gain, programmable Watchdog Timer with Internal Oscillator, an SPI serial port, IEEE std.
1149.1 compliant JTAG test interface, also used for accessing the On-chip Debug system and
programming and six software selectable power saving modes. The Idle mode stops the CPU
while allowing the SRAM, Timer/Counters, SPI port, and interrupt system to continue function-
ing. The Power-down mode saves the register contents but freezes the Oscillator, disabling all
other chip functions until the next interrupt or Hardware Reset. In Power-save mode, the asyn-
chronous timer continues to run, allowing the user to maintain a timer base while the rest of the
device is sleeping. The ADC Noise Reduction mode stops the CPU and all I/O modules except
Asynchronous Timer and ADC, to minimize switching noise during ADC conversions. In Standby
mode, the Crystal/Resonator Oscillator is running while the rest of the device is sleeping. This
allows very fast start-up combined with low power consumption. In Extended Standby mode,
both the main Oscillator and the Asynchronous Timer continue to run.
The device is manufactured using Atmel’s high-density nonvolatile memory technology. The On-
chip ISP Flash allows the program memory to be reprogrammed in-system through an SPI serial
interface, by a conventional nonvolatile memory programmer, or by an On-chip Boot program
running on the AVR core. The boot program can use any interface to download the application
program in the application Flash memory. Software in the Boot Flash section will continue to run
while the Application Flash section is updated, providing true Read-While-Write operation. By
combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip,
the Atmel ATmega128 is a powerful microcontroller that provides a highly flexible and cost effec-
tive solution to many embedded control applications.
The ATmega128 AVR is supported with a full suite of program and system development tools
including: C compilers, macro assemblers, program debugger/simulators, in-circuit emulators,
and evaluation kits.
ATmega103 and
ATmega128
Compatibility
The ATmega128 is a highly complex microcontroller where the number of I/O locations super-
sedes the 64 I/O locations reserved in the AVR instruction set. To ensure backward compatibility
with the ATmega103, all I/O locations present in ATmega103 have the same location in
ATmega128. Most additional I/O locations are added in an Extended I/O space starting from $60
to $FF, (i.e., in the ATmega103 internal RAM space). These locations can be reached by using
LD/LDS/LDD and ST/STS/STD instructions only, not by using IN and OUT instructions. The relo-
cation of the internal RAM space may still be a problem for ATmega103 users. Also, the
increased number of interrupt vectors might be a problem if the code uses absolute addresses.
To solve these problems, an ATmega103 compatibility mode can be selected by programming
the fuse M103C. In this mode, none of the functions in the Extended I/O space are in use, so the
internal RAM is located as in ATmega103. Also, the Extended Interrupt vectors are removed.
The ATmega128 is 100% pin compatible with ATmega103, and can replace the ATmega103 on
current Printed Circuit Boards. The application note “Replacing ATmega103 by ATmega128”
describes what the user should be aware of replacing the ATmega103 by an ATmega128.
4
ATmega128
2467R–AVR–06/08
ATMEL [ ATMEL CORPORATION ]
