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Shield part placement

More brain-dump detail in the forum post on here but suffice to say .. progress is coming well on the shield approach, where the shield is a second board that could be stood on on top of the zikzak rev3 pcb. Decisions (about power supply) have been made and going to go with the general approach that USB power is the way to go, but still supporting power by barrel jack and pinheaders and various combinations thereof; going to go with USB for power, and optional z80 serial over that USB via FTDI chip, with a second USB jack to be supported (calling it 'data usb') hooked up to the USB controller on the GPU. This should let us do crazy stuff like have the z80 tell the gpu to go into USB mass storage mode or other crazyness, if we ever get the time to hack out the code for it :) Schematic has most of the parts on it, and half wired up, and the PCB is getting the placement about half right… so I think I know where the VGA jack will fit in, and where the joystick and keyboard and USB jacks will go.

The image to the left is very WIP (work in progress); its already changed since the picture an hour or two ago, but just to show you where the pads are that bridge down to the ZZ rev3 main pcb, and the difficulty in layout out the jacks. And theres more jacks on there now…

Its made complicated by the fact that the ZZ r3 pcb was meant to go into a 3d printed case itself, with jacks affixed to the case, and those jacks connected to the pcb via jumper wires. Turns out putting jacks (with their mechanical stress during plug/unplug cycles) is hard on the printed plastic, and all those jumper wires just get fiddly after awhile. At anyrate, ZZ r3 pcb was designed for this, so the pin headers are all around the periphery. When you're planning to now put another board on top (or below) this pcb, it in turn would really like the real physical jacks to be around its periphery.. but cannot, since those positions are going to be the pinheaders that bridge down to the ZZ r3 board. In a more ideal world if the intention is for another second board to be the port expander, you put all the pin headers in central elevator-shaft like organization, all in the middle or in convenient location, and then spread out the connectors on the other board. Live and learn :)

My mode of operation tends to be .. given real life is very busy, and I can't bite off too large a piece lest nothing ever get done, to bite off smaller pieces and place one foot in front of the next, with regular achievable milestones. So current goal is to design the shield pcb that goes onto rev 3; rev3 is a real working Zikzak pcb on my bench, now; to make a shield means drawing up all the CAD libary pieces for the jacks and new components (a lot of fiddly work), and the new circuitry for the USB and audio amp and so forth. Some of this has been proven in other little projects or on breadboards, but its a whole other story putting it into a schematic and hooking it all up, and having the schematic pins map to the correct real world pads, and so forth. So, getting a shield laid out, and have the boards made, affords some experience.. it get you to lay pieces out, and you'll prove they work or not. Best case, it all works; next best is it mostly works, and you can work around some minor flaws, but have saved yourself from screwing up on a rev4 very expensive and time consuming board. Or worst case is a glaring flaw you find in shield rev1 and have to make a rev2 of that.. but better to do that, and minimize overall cost (parts and time lost).

So making a shield for rev3 is still the right way to go, as a step to Zikzak rev4 if we go that way (where ZZr4 would be another all in one board like r3, but with the jacks right on board if we can fit it; alternatively, r4 could be a large single board, much like r3, but with all the headers done centrally in some fashion designed for actual second board expansion, and with a few minor corrections.)

So, we prove the shield design, as a revision or two; it'll be fun, and make some fun experiments for ZZ r3 possible. Once thats proven, can decide if going for a ZZ r4 and if its a single or double board solution. Either way, while r4 is in the pipe, can be working on a 3d printed case design for r3, and enjoying r3 more.

Zikzak rev3 will be much more fun, with jacks and the amp and power and all that on one board. As is now, it rather depends on hooking up external power regulation pcb or using a big power supply, and a FTDI external board to do serial communication to host PC, and fiddly external PS/2 keyboard connector and fiddly VGA connector, and a few other connections; which is fine on the bench, but not really convenient to go show someone else at work or on an upstairs TV etc. To make the shield, and put a simple printed case around it, would make it an actual console, plug and play; jack in the USB power, a joystick, and VGA to TV and good to go! Or a keyboard, and cart, and so on…

So, shield for rev3 is coming along very nicely. Then decide what ZZ rev4 will be…


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