Week 4: Fabricating the Fab ISP
Milling the PCB
I elected to make the “David” version of the ISP with a 20 MHZ crystal. I set about sizing and mounting a small piece of PCB material in the Modela mill. I took care to ensure the board was mounted flat with double sided adhesive tape after cleaning up the sacrificial substrate. The 1/64 cutter was mounted with care and offered up to the surface after zeroing the z axis whilst still allowing the small plunge depth needed to remove the surface copper. I loaded the traces bitmap in to the Fab module. Given there was little spare space available on the blank material it was important to check and position the xy origin accurately from the offset in the fab module. Once the datums were set, the cut was executed.
I changed to the 1/32 bit and loaded the interior cutting bit map. The z axis setting was adjusted to ensure there was more plunge depth available this time so as to assure a cut through the board was possible. Then the interior cut was executed.
The resulting PCB was checked for shorts and gently de-burred with a metallic edge. Then it was washed with a mild soap solution.
Adding the components
Populating the board was straight forward following the advice to work from the centre out and to leave tall components to last. Care was needed with the USB connector traces as highlighted and also with positioning the chip to ensure there were no bridges to the paths running underneath it. I checked the board thoroughly for dry joints and unintended bridges and found none.
Programming the ATtiny44
My FabISP was programmed using Joel’s FabISP. We downloaded the firmware zip. The make file needed editing to ensure compatibility with a 20MHz crystal so F_CPU was changed to 20000000. After this modification was made we ran make program.
As my lap top runs 64bit Windows it was also necessary to use the correct device driver. We located this on MightyOhm which linked us on to the correct file on sourceforge.net. Once this was downloaded my computer recognised the FabISP.
