ScoreSure is (almost) pocket-sized!

Following the (let's be honest, rather poor) 100th blog post, I spent a bit of time reading through some of the earlier posts. It strikes me that the earlier posts were a lot more in-depth, as ideas were tried out, and learning explained in a step-by-step basis. Some of the earlier posts are really quite informative, with later posts being more along the lines of "this is what we've done, here's an update".

I think it's time we went back to more informative blog posts, so here's one.
It explains how we got the ScoreSure device off the breadboard and into a pocket-sized device:

After designing the PCB layout, the next stage was to print the layout onto some Press-n-Peel. A hot iron and five minutes later, and the PCB is pressed onto the copper clad board ready for etching.


The etching powder we used this time was Ferric Chloride. It's a brilliant yellow/brown colour which stains everything it touches! On the plus side, it can etch a 4-inch square board in just a couple of minutes (just place the etching container inside a hot water bath and jiggle the board around for 3-4 minutes).


We didn't bother with ground and power planes, so the etchant had quite a lot of work to do! (including ground/power planes is a good idea for two reasons: firstly, nice big power tracks reduce the possibility of dips in voltage; secondly, large areas of exposes copper means less etching is needed - which in turn makes the whole process much quicker).


Here's the etched board after it's been drilled. Each hole (count 'em, there are seventy-four!) was hand-drilled using a PCB drill - although it sounds tedious (it was) it's actually just as quick (or slow) as using a Dremmel and pillar stand - although the actual drilling process is slower by hand, positioning and lining up the drill bit is much quicker/easier without a rotating bit to worry about.


The first thing to do after drilling the board was to install the pin header/sockets along the top and bottom edges of the PCB. Once these were in place, it was possible to check the alignment of the board against the Nokia screen and shield. The photo above shows that only some of the pins from the shield were actually used to connect to the PIC.

Here's the etched and drilled board installed onto the reverse of the Nokia screen shield:


Now that we can be sure that our shield fits onto the back of the Nokia board, it's time to install the components - which consist mainly of an 18LF2455 PIC, 24LC256 eeprom (the L in the serial number in both the PIC and the eeprom means it's a low-power version, running off 2.5v-5.5v) a few discrete components (resistors, capacitors, 20Mhz crystal) and some pin headers.


There are two headers on the board - the one at the top, to the right of the PIC, is the 5-pin ICSP header, which allows us to program the PIC without having to remove it from the board to place into the programmer (which is just as well - the PIC is soldered down!)

The 4-pin header at the bottom is for connecting a USB cable to the PC.
This will enable us to transfer data to/from the eeprom chip once the PIC has been programmed with the appropriate ScoreSure device code.


The plug hacked onto the back of the device was added as an afterthought!
With the Nokia shield in place, there was no way of getting power onto the 5v/ground pins - either via the USB header (it is obscured by the screen when it's in place) or any other means. The small plug allows us to connect the device to a lipo 3.7v battery while the screen is in place, even though it's a bit of a kludge! The +5/ground wires from the plug were soldered directly onto pins 1 and 4 of the USB header (which take 5v/ground from the PC when connected). Obviously in a final design we'll need to isolate this for now it's ok, as long as we remember to disconnect the battery before plugging in the USB cable. (in the final design, we'll probably have a simple switch or maybe even use the reset button somehow, so that the device knows the difference between being plugged into a battery - in "mobile" mode - and being plugged into a PC - in "download" mode - and can enable/disable USB comms accordingly).


So far, so good. Everything looks ok but that's no guarantee that anything will happen when we put power onto this thing. In fact, it's pretty well guaranteed to do absolutely nothing just at the moment - after all, we've not programmed the PIC yet!

Luckily, DonkeyProg recognised the PIC as an 18F2455 and programmed it at the first attempt. Validation went through a treat, so as far as we can tell, the PIC now contains our ScoreSure device code. The last thing to do is connect the PCB to the Nokia shield, attach the battery and see what happens.............