ATmega8 Test Circuit Diagram

This schematic shows the Atmel ATmega8 circuit as it was built on its test board. The ATmega8 is using an external crystal at 16 MHz. There is also an ExpressPCB design file for a full 28-pin AVR development board that supports the ATmega8, and other ATmega microcontrollers. Please note that the ATmega8 is not functionally or software compatible with the x8 family, but it is pin compatible. For instance (from the ATmega8 datasheet):

• There is no divide by 8 clock option, so if you leave the part at it's defaults, using the internal RC oscillator, it will run at 8 MHz, not 1 MHz like the other ATmega microcontrollers.
• It has a maximum 16 MHz system clock. The newer parts will run at 20 MHz.
• The only external interrupts are INT0 and INT1. There are no pin change interrupts.
• The register names are different, so that even though the source code itself might be the same, the register definitions will need to change in order for the code to compile.
• The I/O addresses stop at 0x5f, not 0xff like the newer parts. This moves RAM down as compared to the xx8 series.

The ATmega8 is an old part, and there are "upgraded" versions in the ATmega88, ATmega168 and ATmega328 MCUs. I used the ATmega8 in the serial terminal because it is still the most popular ATmega microcontroller. See a comparison between the ATmega8 and the ATmega88 for some specific differences between them.

Projects featuring the ATmega8

• 28-pin AVR development board
• An ATmega8 + LCD + Keyboard serial terminal

ATmega8 Datasheet

Here you can find the current ATmega8 datasheet

Atmel ATmega8 Pinout

ATmega8 Pinout

Schematic Diagram

ATmega8 MCU simple circuit schematic diagram

This is an ATmega8 circuit with a crystal added. The actual test circuit for the ATmega8 was a serial terminal project using the ATmega8 to read and decode a PS/2 style keyboard and update a 2 x 16 LCD display. Here is some ATmega8 PS/2 keyboard code. There is also a PS/2 keyboard tutorial using an ATtiny4313.