|View schematic and PCB as PDF.|
Preliminary versionI'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 includedThe 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 operationThe microcontroller supply voltage is easily selected by a single jumper setting.
Improved reset circuitA 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 clockI'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 crystalThe 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 circuitryAnalogue power supply follows the advice given in the data sheet.
Temperature sensor now powered from logic outputThe 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 removedI'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 doThere 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.