Its All About Size
Now we know the details of what we want to do, next we need to know what size and shape its going to be. What you can plug into the ST cartridge port is limited in nearly every dimension, so if it’s going to fit, it needs to be right!
First of all there is the edge connector. The cartridge port female edge connector has a 41mm wide slot with 20 connecting pins per side at a pitch of 2mm. This will accept a 1.6mm thick PCB (which is a standard thickness) which sits inside the connector to a depth of about 10mm. So we will need to create a suitable male edge connector footprint for this and make sure its setup correctly in the board outline.
Next, the width of the cartridge shell which can be plugged into the machine is also limited to a minimum width of 46mm (inner dimension) and maximum of 58mm (outer dimension). So we will need to make sure the PCB will fit within the cartridge shell (which we will also be making). The limited space above and below the cartridge connector will also be important, as this will determine where we can put taller components (like flash ROM’s in sockets). We have roughly 7mm above and 14mm below the PCB, so any larger components will need to be on the bottom.
So finally we can start getting a feel for where components are going to go. The ROM’s are physically quite large as the smallest flash ROM I could find was 128K, which is larger than we need — so I laid these side by side longways up the cartridge. This was the main limiting factor for the rest of the board, as it defines the minimum width (which wasn’t far off the edge width) and also where I could add a hole to screw the shell together. The other components, being small surface mount parts, fit relatively easily the other side of the hole and define the length of the PCB.
The next thing I did was to make up a prototype cartridge shell to see if the shell would fit correctly with my measured dimensions. For this I used SketchUp to model, and I have a DaVinci 3D printer for printing.
As it turned out, it was a little small in the width and a little large above and below the connector, so I tweaked this and had something which seemed pretty good.
This had the knock-on effect that the PCB could be a few millimetres wider due to the larger case, which was actually quite handy as space was quite tight for the ICSP header.
As getting your PCB’s manufactured takes several weeks if you’re doing it on the cheap via a Chinese fab house, it really pays to take the extra time and try and get it all right the first time!
While we wait for the PCB’s to come back from the fab house, the next thing to do is to write some code to go on the ROM’s ready for when they arrive.