Powering the ConnDuino can be achieved:
- through a 5.5x2.1 barrel jack (6V to 16V)
- through a block terminal (6V to 16V)
- through the FTDI pins
- through the VIN pin (up to 5.5V).
ConnDuino departs from the Arduino reference design in its ability to power the Atmega328 microcontroller, the brains of the board, using either 5V or 3.3V. Selection is possible by placing the “VCC-SEL” jumper, located at B, in the appropriate position. Powering, the microcontroller with 3.3V may be an interesting option, when most of the connected devices operate strictly on 3.3V logic. More and more sensors and devices are doing so. Raspberry Pi success may have contributed to this trend, since it uses 3.3V logic. Surely, it sounds great to interchange devices between different platforms. There is a catch however. The microcontroller voltage is directly related to its clock frequency. For 3.3V the datasheet suggests 12 Mhz clock maximum. Typically, a lower value should be used, say 8~10MHz. Surprisingly though, others have reported in the official forums, 16Mhz/3.3V operation! This topic surely deserves further experimentation.
Another interesting option offered by ConnDuino, is the selection of I2C (2-wire interface) voltage, by placing the “I2C-SEL” jumper, located at B, either at 5V or 3.3V. Also, the external pull-up resistors R5 and R6 must be installed for this setting to work (otherwise the I2C bus voltage is determined by the microcontroller voltage). See in this article about the external and internal pull-up resistors and their values. With all these, the microcontroller may operate at 5V and the I2C devices at 3.3V. Such a combination allows running the microcontroller at full speed, while still safely using 3.3V devices on I2C bus (sd-cards, sensors etc).
The supply circuit (located in the area A of the last picture), offers over-current and reverse polarity protection. A ptc resettable fuse is installed (rated at 500mA). The input voltage is regulated by an AMS1117-5 and an AMS1117-3.3 regulator (rated at 1A), providing two distinct power rails: one at 5V and the other at 3.3V. External sensors and modules may be powered by either voltage. This feature is present in the official Arduino Uno board, however in ConnDuino, as many as six regulated voltage pins are available (four at 5V and 2 at 3.3V), together with six ground pins. Looking at the picture above, these are located in area B. Furthermore, the official Arduino Uno specifications limit the 3.3V output current to only 50mA. In ConnDuino, the 3.3V power rail is equivalent with the 5V one, and both are limited by the 500mA fuse. This is essential since many TFT’s that operate at 3.3V, are power hungry.
Additional information regarding the ConnDuino board design and functionality can be found in the following articles: