Monday 18 June 2012

Calunium v2.0: Preliminary design

View schematic and PCB as PDF.

Preliminary version

I'm planning to release an improved version of Calunium, my Arduino clone. Before finalising my design I'd like to have some feedback from other ATmega1284 users and would-be users. I make no promises that your suggestions will be included but I value any constructive feedback you give.

Summary of changes

Arduino 1.0 headers included

The revised Arduino headers have been included. This adds dedicated I2C connections and an IOREF pin whose purpose is to indicate the I/O voltage level.

Jumper-selectable 3.3V or 5V operation

The microcontroller supply voltage is easily selected by a single jumper setting.

Improved reset circuit

A reversed-biased diode has been added in parallel with the reset pull-up resistor to prevent overshoot of the reset signal which can cause erroneous behaviour.

Option to use surface-mount real-time clock

I've not been able to find a suitable alternative to the DS1307 which can operate at 3.3V and which is available in a dual inline package. The alternatives are only available in SOIC8 or smaller packages. To support 3.3V operation a combined DIP/SOIC8 footprint has been used.

Ceramic resonator option instead of crystal

The crystal footprint has been revised to allow a 3-pin ceramic oscillator to be fitted as an alternative to a crystal. Omit the 22pF loading capacitors when using a ceramic oscillator.

Improved analogue power supply circuitry

Analogue power supply follows the advice given in the data sheet.

Temperature sensor now powered from logic output

The optional LM61 (or similar) temperature sensor is now powered from D7. This change has been made for low power operation; I've been fitting an LM61 to other shields instead of using one on Calunium so that it can be powered off when not needed.

V-USB removed

I've had no success with the V-USB interface so the necessary circuitry has been removed. If you have had any success please let me know how to make it work! The USB connector remains as a convenient 5V power connector.

To do

There a few things to do that I can't at this stage:
  • Add white silkscreen areas on underside of the PCB for writing the serial number and crystal frequency.
  • Clean up silkscreen text.

Anything else?

There is approximately 0.7" × 1" unused space on the PCB. What should be included that is currently missing?
  • Ciseco XRF module? Probably not enough space for the jumper settings I'd like to include, and planned for another board.
  • Add a 5V regulator connected to Vin? Without a separate power connector (too bulky and infrequently used?) I'm not sure it is very useful.

The Eagle PCB design files for Calunium v2 are available on Github and are licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License.

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.