Sunday 17 June 2012

Options for powering Calunium

Calunium with power adaptor fitted
Calunium with 3.3V prototyping power adaptor fitted. Click on the image for an annotated version.

Why there isn't a 5V regulator

One of the reasons why I designed Calunium instead of using the Arduino Mega2560 was the inefficient linear power regulators on the Mega2560 prevent low-power battery operation. I considered various high-efficiency low-dropout regulators (the MCP1702 series is a favourite) but decided that as there are so many possible power options (for example, USB, NiMH, alkaline or lithium polymer batteries or solar cell) I would omit the 5V regulator and include a dedicated power connector. I expect that any embedded use of Calunium will require an additional circuit board and that is the best place to locate power supply circuitry appropriate to that particular use.

3.3V or 5V operation

One of the key features I wanted was to allow either 3.3V or 5V operation. By using the auxillary power connector Calunium can easily be powered at 3.3 or 5V. To make protyping easier I've added the appropriate regulators to two small pieces of stripboard, either of which can be placed directly onto the auxillary power connector. Depending on which stripboard is connected the microcontroller operates at either 3.3V or 5V.

Calunium power adaptors
Calunium power adaptors for prototyping; 3.3V (top) and 5V versions. Click on the image for an annotated version.

Selectable 3.3V or 5V operation

For selectable 3.3V or 5V operation omit the onboard 3.3V regulator. Also omit the polyfuse so that there is no danger of connecting the 5V from the USB or FTDI connectors to the microcontroller or peripherals. USB power is still available on the auxillary power connector. Fit a 12MHz crystal, at 3.3V operation 16MHz is outside of the safe operating area for the microcontroller but 12MHz operation is within the specifications. For the lowest power operation omit the LEDs and 2N7000 FET.

3.3V operation

Link the 3V3 and 5V connectors and provide 3.3V to both of them. Note that the DS1307 requires 4.5 to 5.5V for proper operation. The only suitable replacements for the DS1307 that I've found are surface mount SOIC8 packages (eg DS1338, MCP79410, MCP79411, MCP79412) which then requires a SOIC8-DIP adaptor. A newer revision of Calunium is planned which will contain both SOIC8 and DIP8 footprints for the real-time clock.

5V operation

Provide 5V to the 5V connector, from either the USB power supply (VUSB) or an external supply. If the 3V3 rail is used then an external 3.3V regulator must be provided; some shields (the Ethernet shield and my RN-XV+SD shield) have their own 3.3V regulator anyway since the standard Arduino board can only provide 50mA to the 3.3V rail. If the 3.3V regulator is powered from the 5V supply a low-dropout regulator will be required.

Coin cell operation (3V)

The onboard CR2032 or CR1220 coin cell is normally used for battery backup of the real-time clock. To use it as a power source for the microcontroller follow the instructions for selectable 3.3V or 5V operation and then connect the VBAT connector with the 3V3 and 5V connectors. Note that coin cell operation is only suitable for very low power uses. Fit an 8MHz crystal to ensure the microcontroller operates within its specifications.

External power

If an external power source is used, such as a 9V mains adaptor, the input supply can be connected to VIN for distribution to the shields.

Future improvements

When Calunium was designed it wasn't clear how 3.3V operation should be implemented, particularly when shields are used. I took the approach of providing 3.3V to the 5V rail and ensuring my RN-XV+SD shield could operate at either voltage. Following the release of the new Arduino Leonardo boards with their extended connector featuring IOREF a better approach is possible: keep the 3.3V and 5V rails at the designed voltages and power shields from the IOREF connector. This is the approach I am taking with the next version of Calunium as it enables voltage selection to be made using a single jumper.

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