I've been working on a car project, starting with an Atmega328P Arduino and quickly realizing that the 32KB of flash and 2KB of SRAM were not going to cut it, and so I decided to use an Atmega1284P. I decided to design my own board which would suit my needs. Now that I have received the first batch of PCBs, put one together and tested it, I wanted to see if there's any interest for such a board in the Arduino and DIY community, since support for the Atmega1284P can be added to the Arduino IDE quite easily.
Key features of this board are: 1) Breadboard compatible. It fits into a standard breadboard leaving 1 column on one side and 2 columns on the other side. It has 2x19 pin headers. All 32 IO pins of the Atmega1284P are exposed. 2) V-USB circuitry is integrated. Pins PB0 and PB1 were chosen for this purpose. 3) Powered from USB. 5V can also be supplied by connecting it to the VCC pin. I'm also considering making a daughter board with a DC jack and a 5V regulator to provide external power, if there's a demand for it. 4) Onboard 0.8 - 1A 3.3v LDO voltage regulator. Since the only heatsink is a small area of copper on the PCB itself, the continuous current draw limit will be much lower than 1A. 5) A ferrite bead for extra filtering of the voltage going to the analog power supply pin. 6) A 3.3v bi-directional level-shifter for the I2C (with built in pullups), uni-directional SPI (MOSI, SCK, SS) and UART (PD3/TXD1, PD4/XCK1) buses, pin PB3 is also level shifted to 3.3v as outputs only, this way up to 4 3.3v SPI slave devices can be used. 7) USBASP bootloader so that sketches can be flashed without an external programmer, right from Arduino IDE. 8 ) An extra tactile switch to activate the above mentioned bootloader. 9) A 16MHz or 20MHz crystal can be used (tested with 12MHz, as that the only xtals I have at the moment). 10) A 500mA polyfuse. A Schottky diode is used to protect the USB power supply against external power supplies. 11) The ISP header can also be used as an SPI header. This is controlled by a jumper. 12) Power and USB activity LEDs. 13) A pin with a 1Kohm pulldown resistor. * The above specifications are for the next board revision.