Hmm, an Atmel-ISP... Don´t know how many of these lay arround here, i think 4. But lets make a fifth...
I don´t want to invent the wheel again, so i looked for an ISP from the last years class and found the "FabTiny*ISP"
It´s a nice little layout for an ISP, using a Tiny45 and double-use the reset pin. This way no jumper is needed. Basicly it is perfect. But i found a little feature to improve it: in the original version you need a ribbon-cable to connect the ISP to your circuit. I don´t like these cables. They are always 1cm to short, no matter how long the cable is.
The cable breaks, longer than 30cm brings problem, and debugging is a pain. So my ISP will have a female-pin-header on the short side...
This way you can put the ISP directly onto the circuit and can use a standard-usb-cable up to 5m for the connection to your PC.
Let´s go... I´ve downloaded the eagle-schematics from the FabTiny*ISP-Page and refined them a little bit.
The old design uses big 1206 resistors, the layout didn´t allowed my second pin-header,
and for debugging-reasons (or to repair a common failure while programming) the XTAL1 and XTAL2 Pins of the Tiny45 was routed to two extra-pins on the ISP-Connector. This is not bad, for the case you bricked the Tiny with a wrong oscilator-selection.
Well, i hope i don´t make this mistake and made a 6-Pin-Connector out of the 8-Pin.
The resistors were replaced by 0402-packages, the two capacitors by 0603. This way i could make the circuit-board a little bit smaller.
Ah, the USB-Connector... The old design uses the PCB as USB-Connector. It is a nice idea, but i don´t work with the most USB-Ports due to the heigt of the PCB. So replaced this by a micro-USB-connector. Smaller and more reliable...
After the layout comes the building... Luckily we have a PCB-Mill in our Lab, a "S63" from LPKF. Price around 25.000$, but
it is a dream to work with it. Simple steps:
After milling it its a good idea to scrub of the PCB to remove old fingerprints and stuff. After this i applied some solder resist
and checked the board under the microscope.
To connect the vias i used a "Favorit" rivet-press from Bungard: "Bungard"
At the next step i used another machine from LPKF, a Pick-and-Place. This
machine is basicly a X-Y-Machine with a air-actuator. You can dispense and place parts very fine-pitch.
So: first dispense solder-paste on the Board, then place the Surface-mounted-parts into the paste.
At this step i didn´t placed the switches and pin-headers. I don´t know if these plastic-things likes the heat in the reflow-oven...
After 3 minutes at 200°C it looks fine :)
Just placing the switches and solder it by Hand, and voila, the finished "FabVeryTiny*ISP"