speccery Posted December 22, 2022 Share Posted December 22, 2022 (edited) Updated 2022-12-30: The board works! Only GROM support tested so far. Original message: I have several Raspberry Pi Pico microcontroller boards which I find quite adorable as they are very affordable, in some respects very powerful and in other respects not too much so. Nevertheless I have been playing around with them for a while and trying various things, I built a prototype eurorack synthesiser sequencer with one for example. I've also used them with some boards I bought from Pimoroni to drive VGA screens and DVI monitors. In the last pandemic call which I participated (I guess around a week and a half ago) we also talked about them. Anyway it was high time for me to get one connected to a TI-99/4A. I know that other folks such as @arcadeshopper and @jedimatt42 have also contemplated the same idea. Perhaps someone has already done it, but if so I'm not aware of it and anyway making boards is fun when there is time for it. So I designed a prototype board and just submitted an order for them. I did not quite have enough time to really do this well, so I decided what the heck, let's see what happens with a quick design. It's a bit bulky, and will not fit into an ordinary cartridge case. This of course also means that I have no idea if this is going to work for sure, but of course I have reason to believe it will. It is not too different design-wise from the StrangeCart, except that the microcontroller is different. The RP2040 chip which powers the Raspberry Pi Pico is not 5V tolerant, so I threw in four 74LVC245 buffer chips and some other stuff. Since the board is large, I used some of the extra space to add a prototyping area, and I added breakouts for three SOT-23-5 footprints and two extra SOIC-8 footprints as well. One SOIC-8 footprint, U5, is fully connected to a SPI port and can serve as flash memory expansion or alternatively house a PSRAM (pseudo static RAM chip). As can be seen from the pictures, the buffers are through hole components, the idea being that if I get it to work and others are interested in these, it would be quite easy for people to build these. I don't like through hole resistors or capacitors, so I used 0805 sized surface mount parts for those. In my opinion they are easy to work with. The only slightly challenging part to solder for some could be the 74LVC1G125 buffer I put in to drive the GREADY signal. It can be replaced with a transistor circuit in the prototype area. To get quickly up to speed, I used FlashGROM99 board as the basis. I removed everything else except the outline, regulator and the edge connector and started from there, to have a basis for the design in Kicad. After placing the parts it became obvious it would be hard to make this fit inside a cartridge housing, so I just gave the board more space which made it easy to route the signals to the Raspberry Pi Pico. Now I just need to wait to get the boards and hope for the best during bringup... Edited January 3 by speccery Added pictures of actual board. 22 1 Quote Link to comment Share on other sites More sharing options...
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