The RKAT28sb is an Arduino compatible ATMEL development board.

PWR IN
9-12VDC

0V
0V           PC5  AD5 SCL
5V           PC4  AD4 SDA
RX                   AREF
TX                    GND
DTR          PB5 D13  SCK
             PB4 D12 MISO     
RST          PB3 D11 MOSI
+3V3            PB2   D10
+5V             PB1    D9
GND             PB0    D8
GND
VIN             PD7    D7
                PD6    D6
AD0 PC0         PD5    D5
AD1 PC1         PD4    D4
AD2 PC2         PD3    D3
AD3 PC3         PD2    D2
AD4 SDA PC4     PD1 D1 TX
AD5 SCL PC5     PD0 D0 RX

        246   RESET
        135
        ISP
        
NB ISP orientation is not Arduino UNO compatible

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 its 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.

This kit doesn't come with a chip and therefore no boot loader. The processor I bought was blank but has been programmed with Optiboot.

A 6-pin ATMEL ISP header is present. This will require an ISP adapter for the USBasp.

D12 MISO PB4 |1 2| VDD
D13 SCK  PB5 |3 4| D11 MOSI PB3
RESET        |5 6| GND

Constructors guide

Schematic