This is a simple Open Hardware peripheral for the OLPC XO-1/1.5/1.75/4.

The files are created using Eagle 6.2.0 on Linux and the SparkFun footprint libraries.

Some details about the design:

1. Supporting using either the ATtiny85 (lowest cost) or the ATtiny861 (14 I/Os, slightly more expensive).

2. Based on the Sparkfun AVR Stick http://www.sparkfun.com/products/9147 and thus software will be based on http://www.obdev.at/vusb/ Note that the USB port has been moved to PB3/4 so software for the SparkFun AVR Stick will not work without modification.

3. The PCB we're working with is 0.8mm (0.031"); a USB plug "is supposed to be 2.4mm (0.093") but you can get away with 1.6mm (0.062")" (thanks, djgpp) Our solution is to put two breakaway tabs on the back of the board. You can break them off and glue them below the USB fingers to make it stay in the port better/at all. One gotcha is that break away PCBs tend to stress nearby traces; I've tried to give them adequate clearance. We're also putting copper fills on the PCBs to make them thicker (thanks again, djgpp). The PCB we have also has 4 layers, but I've routed this board with only the top and bottom so that 2 layer reproductions will work just fine as well. The inner two layers are ground and power planes, which don't need to be present to connect the ground and power nets.

4. I'm attempting to bring BOM below $1/student and part count down as low as possible as well. As a result, we're skimping a bit on ferrites and bypass caps.

5. There are pads for populating a mini USB B connector, so you can put this on a cable (and avoid the glue-the-PCBs-together dance).

6. The XO Stick has a standard arduino shield pinout. This lets it be piggybacked on another XO Stick for programming, or piggy backed on an XOrduino to control a turtle robot. Standard arduino shields will probably not work, since not all pins are connected -- but they might!

7. To program one XO Stick from an XOrduino, the RST header on the target should be put in the "PROG" position so that the programmer can drive it via arduino pin 0. To program an XO Stick from another XO Stick, both RST headers should be in the "NORM" position, and the programmer needs to have the RSTDISBL fuse programmed, so that it can use PB5 ('85) or PB7 ('861) as an output. Note that once RSTDISBL is programmed, the device can't be programmed using the ICSP port until RSTDISBL is unprogrammed using the high-voltage programming method.

8. The XO Stick currently needs an external programmer. Can we write a USB DFU bootloader using v-usb? Something like:

    http://www.obdev.at/products/vusb/usbasploader.html
    http://embedded-creations.com/projects/attiny85-usb-bootloader-overview/
    https://github.com/embedded-creations/USBaspLoader-tiny85
   

-- C. Scott Ananian, 2012-06-09; revised 2012-07-30, 2012-08-08