3. A-Star 328PB Micro pinout and components

These files document the hardware design of the A-Star 328PB Micro:

The pinout diagram identifies the I/O and power pins on the A-Star 328PB Micro, and is shown below.

The A-Star 328PB is based on the ATmega328PB AVR microcontroller from Atmel (now part of Microchip), and most pins on the board are directly connected to the microcontroller. The A* circuit board has printed indicators that you can use to quickly identify each pin’s capabilities: pins labeled with A0 through A7 can be used as an analog input, and a square wave symbol under the pin number means it can be used as a PWM output. All of the pins are labeled on the bottom silkscreen, and some of the pins are also labeled on the top silkscreen (as space allows). For more information about each pin’s capabilities, see Microchip’s ATmega328PB documentation.

Note that the pins labeled SDA1 (PE0) and SCL1 (PE1) are not limited to being used for TWI/I²C and can be used as general-purpose I/O pins. These pins are new on the ATmega328PB and have no equivalent on the ATmega328P, so they do not yet have official pin numbers in the Arduino environment; therefore, we recommend using the constants SDA1 and SCL1 instead of the pin numbers 22 and 23 in Arduino code. (See Section 4.4 for more information on ATmega328PB support in the Arduino IDE.)


The A-Star 328PB has two indicator LEDs:

  • A yellow user LED in the lower right is connected to Arduino digital pin 13, or PB5. You can drive this pin high in a user program to turn this LED on.
  • A power LED in the lower left indicates when the logic supply voltage, VCC, is present. The color of the power LED depends on the version of the board:
VCC Frequency Power LED color
5 V 16 MHz Blue
5 V 20 MHz Red
3.3 V 8 MHz Green
3.3 V 12 MHz Yellow


The A-Star 328PB includes a 6-pin header along its top edge that can be used for TTL serial programming with the preloaded Arduino-compatible bootloader. The serial interface is compatible with our USB AVR Programmer v2.1 and commonly available USB-to-serial adapter cables (e.g. FTDI cables). See Section 4.2 for more information about the serial interface. The TTL serial header can also provide power to the A-Star through the VCCIN pin, as detailed below.

The board also has a 6-pin ISP header that allows it to be programmed with an external AVR in-system programmer, such as our USB AVR programmer v2.1. Pin 1 of the header is indicated with a small dot and has an octagonal shape.

A-Star 328PB Micro with included header pins soldered for breadboard use.

A-Star 328PB Micro connected to the serial pins of a Pololu USB AVR Programmer v2.1.


The main power input for the A-Star 328PB Micro is the BAT+ pin, which is reverse-protected and can accept power sources up to 15 V. Reverse-protected power can be supplied to other devices by using the VIN pin as an output. The VIN voltage feeds into a 100 mA low-dropout (LDO) regulator to provide a regulated 3.3 V or 5 V logic supply called VCC. Alternatively, the external supply can be connected directly between VIN and GND, bypassing the reverse-voltage protection.

The VCCIN pin on the A-Star 328PB Micro serial header functions as a secondary power input for VCC that bypasses the regulator. To avoid unexpected behavior, it is important for the VCCIN voltage to match the regulator voltage on the version you are using. (Supplying a higher voltage to VCCIN raises the logic voltage of your system, and supplying a lower voltage can result in a brownout reset and/or violate the maximum frequency vs. VCC specifications in the ATmega328PB datasheet.)

The A* board includes a switching circuit that makes it safe to have power supplies connected to both VCCIN and BAT+ at the same time, but we do not recommend connecting multiple power sources when the board is powered through VIN instead of BAT+, as the switching circuit does not prevent current from flowing into or out of VCCIN in that situation.

Related Products

A-Star 328PB Micro - 5V, 16MHz
A-Star 328PB Micro - 5V, 20MHz
A-Star 328PB Micro - 3.3V, 8MHz
A-Star 328PB Micro - 3.3V, 12MHz
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