The RKAT28sb is an Arduino compatible ATMEL development board.

PWR IN
9-12VDC

0V
0V                AD4 SDI
5V                AD4 SDA
RX                   AREF
TX                    GND
DTR               D13 SCK
                 D12 MISO     
RST              D11 MOSI
+3V3                  D10
+5V                    D9
GND                    D8
GND
VIN                    D7
                       D6
AD0                    D5
AD1                    D4
AD2                    D3
AD3                    D2
AD4 SDA             D1 RX
AD5 SCI  246  RESET D0 TX
         135
         ISP

This device can be powered by either the pins on the serial port or via the DC input jack. In either case the MCU can be provided with 3V3 or 5V. The jack feeds two LDO regulators which may be set via a jumper to select the required voltage.

The kit comes with a 16M resonator but I have installed a 16 MHz XTAL. The MCU is not rated to run at 16M using 3V3 but this configuration works fine for me.

A cp2104 serial module is well suited to connect to the RKAT28sb with it's built-in capacitor on DTR and is able to supply both 3V5 and 5V for VCC. The photo. top right, shows a cp2104 module connected.

Constructors guide with schematic