Arduino Pro - 3.3V/8MHz (ATMEGA328)

Arduino Pro - 3.3V/8MHz (ATMEGA328)
Item# ARD-PRO-3

Product Description:

Arduino Pro - 3.3V/8MHz (ATMEGA328)
This is a 3.3V Arduino running the 8MHz bootloader. This board does not work with the 5V Arduino shields but does work nicely with the prototyping shield.

Please note: The right-angle power switch is fragile. Please be gentle.

Arduino Pro does not come with connectors populated so that you can solder in any connector or wire with any orientation you need. We recommend first time Arduino users start with the Nano or Duemilanove. They are great boards that will get you up and running quickly. The Arduino Pro series is meant for users that understand the limitations of system voltage (3.3V), lack of connectors, and USB off board.

Arduino Pro designs to be low profile. The DC power jack footprint is available, but not populated. It is recommend running the board with a LiPo battery for better portability.

The Arduino Pro required an FTDI cable or the Arduino LilyPad/Pro/ProMini Programmer to download the program.


ATmega328 running at 8MHz external resonator
Low-voltage board needs no interfacing circuitry to popular 3.3V devices and modules (GPS, Accelerometers, sensors, etc)
USB connection off board
3.3V regulator
Reverse polarity protected
DC input 3.3V up to 12V
Resettable fuse prevents damage to board in case of short
Power select switch acts as on/off switch


The Arduino Pro is a microcontroller board based on the ATmega328 (datasheet). It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, an 8 MHz resonator, a battery power jack, a power switch, a reset button, and holes for mounting a power jack, an ICSP header, and pin headers. A six pin header can be connected to an FTDI cable or Sparkfun breakout board to provide USB power and communication to the board.

The Arduino Pro is intended for semi-permanent installation in objects or exhibitions. The board comes without pre-mounted headers, allowing the use of various types of connectors or direct soldering of wires. The pin layout is compatible with Arduino shields. The board can be powered with a battery, and runs at 3.3V.

The Arduino Pro was designed and manufactured by SparkFun Electronics.

Schematic & Reference Design

EAGLE files:

Schematic: Arduino-Pro-schematic.pdf


Operating Voltage3.3V
Input Voltage3.35 -12 V
Digital I/O Pins14 (of which 6 provide PWM output)
Analog Input Pins6
DC Current per I/O Pin40 mA
Flash Memory32 KB (of which 2 KB used by bootloader)
Clock Speed8 MHz


The Arduino Pro can be powered via the USB header, with a battery, or with an external power supply. The battery power jack is a JST header. A power jack for an external supply can be soldered to the board.

The power pins are as follows:

  • Batt. The voltage supplied to the board from the battery connector. You can supply voltage through this pin, or, if supplying voltage via a battery access it through this pin.

  • VCC. The raw (unregulated) voltage being supplied from either the battery or the external power supply.

  • 3V3. The regulated 3.3 volt supply.

  • GND. Ground pins.


The ATmega328 has 32 KB of flash memory for storing code (of which 2 KB is used for the bootloader). It has 2 KB of SRAM and 1 KB of EEPROM (which can be read and written with the EEPROM library).

Input and Output

Each of the 14 digital pins on the Pro can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead() functions. They operate at 3.3 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms. In addition, some pins have specialized functions:

  • Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are connected to the TX-0 and RX-1 pins of the six pin header.

  • External Interrupts: 2 and 3. These pins can be configured to trigger an interrupt on a low value, a rising or falling edge, or a change in value. See the attachInterrupt() function for details.

  • PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output with the analogWrite() function.

  • SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication, which, although provided by the underlying hardware, is not currently included in the Arduino language.

  • LED: 13. There is a built-in LED connected to digital pin 13. When the pin is HIGH value, the LED is on, when the pin is LOW, it's off.

The Pro has 6 analog inputs, each of which provide 10 bits of resolution (i.e. 1024 different values). By default they measure from ground to VCC, though is it possible to change the upper end of their range using the AREF pin and some low-level code. Additionally, some pins have specialized functionality:

  • I2C: 4 (SDA) and 5 (SCL). Support I2C (TWI) communication using the Wire library (documentation on the Wiring website).

There are a couple of other pins on the board:

  • AREF. Reference voltage for the analog inputs. Used with analogReference().

  • Reset. Bring this line LOW to reset the microcontroller. Typically used to add a reset button to shields which block the one on the board.

See also the mapping between Arduino pins and ATmega328 ports.


The Arduino Pro has a number of facilities for communicating with a computer, another Arduino, or other microcontrollers. The ATmega328 provides UART TTL serial communication, which is available on digital pins 0 (RX) and 1 (TX). The Arduino software includes a serial monitor which allows simple textual data to be sent to and from the Arduino board via a USB connection.

A SoftwareSerial library allows for serial communication on any of the Pro's digital pins.

The ATmega328 also supports I2C (TWI) and SPI communication. The Arduino software includes a Wire library to simplify use of the I2C bus; see the documentation on the Wiring website for details. To use the SPI communication, please see the ATmega328 datasheet.


The Arduino Pro can be programmed with the Arduino software (download). For details, see the reference and tutorials.

The ATmega328 on the Arduino Pro comes preburned with a bootloader that allows you to upload new code to it without the use of an external hardware programmer. It communicates using the original STK500 protocol (reference, C header files).

You can also bypass the bootloader and program the ATmega328 through the ICSP (In-Circuit Serial Programming) header; see these instructions for details.

Automatic (Software) Reset

Rather then requiring a physical press of the reset button before an upload, the Arduino Pro is designed in a way that allows it to be reset by software running on a connected computer. One of the pins on the six-pin header is connected to the reset line of the ATmega328 via a 100 nanofarad capacitor. This pin connects to one of the hardware flow control lines of the USB-to-serial convertor connected to the header: RTS when using an FTDI cable, DTR when using the Sparkfun breakout board. When this line is asserted (taken low), the reset line drops long enough to reset the chip. The Arduino software uses this capability to allow you to upload code by simply pressing the upload button in the Arduino environment. This means that the bootloader can have a shorter timeout, as the lowering of the reset line can be well-coordinated with the start of the upload.

This setup has other implications. When the Pro is connected to either a computer running Mac OS X or Linux, it resets each time a connection is made to it from software (via USB). For the following half-second or so, the bootloader is running on the Pro. While it is programmed to ignore malformed data (i.e. anything besides an upload of new code), it will intercept the first few bytes of data sent to the board after a connection is opened. If a sketch running on the board receives one-time configuration or other data when it first starts, make sure that the software with which it communicates waits a second after opening the connection and before sending this data.

Physical Characteristics

The maximum length and width of the Pro PCB are 2.05 and 2.10 inches respectively, with the six pin header and power switch extending slightly beyond the edges. Four screw holes allow the board to be attached to a surface or case. Note that the distance between digital pins 7 and 8 is 160 mil (0.16"), not an even multiple of the 100 mil spacing of the other pins.

NOTE: Pictures below show ATMEGA168. It is now shipping with ATMEGA328.