The ATmega644 is often used as a "bigger & badder" Arduino. There are bootloaders available for it, and Arduino extensions that support it.
There are three versions - the ATmega644, which has typical speed and voltage modes, the ATmega644V, which is a reduced voltage/speed version, and the ATmega644A/PA which has an additional USART and 7 more interrupts. The ATmega644V will run from supplies as low as 1.8V, but it's maximum frequency is reduced from 20MHz to 10MHz.
The calculator below provides a way to find the maximum operating frequency for any valid power supply voltage (2.7 to 5.5). Enter the power supply voltage and click "Calculate Max Frequency" to find the ATmega644's specified maximum clock speed at your Vcc.
|RT Counter (w/osc)||1||1|
|Power Consumption (approximate)||1.125mA/MHz|
|Maximum I/O Current (per pin)||40mA|
|Maximum I/O Current (all ports)||100mA(low)/150mA(high)|
|Maximum I/O Current (total)||200mA (PDIP)|
|Maximum I/O Current (total)||400mA (PDIP/QFP/MLF)|
A minimal ATmega644 circuit consists of an ATmega644. Everything else is optional. The circuit below shows a programming port, reset circuit, and a crystal oscillator. Any or all of these could be left off and the device would function perfectly well. By default the unit runs from the 1MHz internal RC oscillator.