In some upcoming projects I intend to embed some processing intelligence into small devices. The smaller Arduino boards are too big and expensive for these applications.
After some investigation, I settled on using the ATTiny series of 8 pin microcontrollers. These processors vary in capability (from a very low end) and all provide 6 I/O ports. Tools compatible with the Arduino ecosystem are also available.
As a first step, I designed a small breakout board for the SOP8 version of these MCUs.
My design goals were relatively simple:
- Make the board as small as possible and still be practical to use.
- Include sensibly sized solder pads for wires and/or other headers.
- Include a programming interface on the board.
Minimizing size meant that the only component on the breakout board is the SOP8 package, so it was straightforward to create an Eagle schematic to fit requirements. As most, if not all, the 8 pin Atmel processors chips are pin compatible, this design also provides the flexibility to choose the MCU that suits the final application. The schematic and PCB files can be downloaded here.
An important consideration with the SOP8 ICs was to have some way of programming the MCU. Unlike the DIP package, these cannot be mounted in a socket for removal and programming. At the cost of a making the PCB bigger, the board includes holes for a 6 pin ICSP header.
Finally, implementing the board in a system means that wires will need to be attached to it, so standard sized and spaced pads make this job easier.
The resulting PCB design was approximately the size of my thumb’s fingernail (dimensions on the left are in millimeters).
As most PCB fabrication houses price to a minimum 5cm square size, I decided to panelize the board as 2×2. This was my first time panelizing so I Googled “how-to” and followed the instructions. The output yield improved from ten ‘boards’ to forty, which I expect will be lasting me a long time!
The PCBs were manufactured by DirtyPCBs. The panelized boards worked fine except for the silk screening – lesson learnt for next time I use this service.
The MCU ICs were soldered on before separating the individual boards, as the panelized board was easier to handle. The first batch of 4 boards are using ATTiny13A MCUs, as they are dirt cheap and provide enough capability for the simple applications I have planned.
One of the breakout boards was made breadboard friendly by attaching the 6 pin ICSP header and the I/O pin headers. In this configuration it fits neatly over the central gap of most breadboards and makes it easier to experiment and prototype applications.