I don't know if this is something that the world needs, or if it is even going to work, but I just finished the schematics and PCB design for my own take on the "mini/nano/whatever/quark" version of PEB. Since it's my version, I decided to call it simply Erik's Tiny PEB, as this is supposed to be small(ish).
I attach the schematics and current PCB layout. The schematics and the board are a bit of a mess, since the design is based on my existing and working "SD processor board", I just added a few things. I didn't want to layout everything again, so at places the board is dense and I haven't cleaned up the schematics.
Since the design is partially based on my existing work, theoretically this should work. The additions are:
- 512K SRAM
- 4MB SPI FLASH
- TI-99/4A side port connector (created an Eagle symbol for it, hopefully that went ok)
Once I've verified the PCB and schematic design I'm going to send this off to China for PCB production. This should enable me to build a working prototype (hopefully).
The board is supposed to enable the following once it works (the way I imagine it - there are programmable components here and I don't know how much stuff will fit in, and it is hard to test without a prototype):
- SD card storage support, with support for FAT, FAT32, SD and SDHC cards
- 256K of paged RAM memory (and of course the standard 32K as a subset). I'm not sure if the standard paging will fit into the CPLD, but I think it will as I am not planning to store the page entries on the CPLD, but actually on external SRAM.
- 256K of additional RAM which needs to provide support for DSR routines, potentially also for ROM cartridges and maybe even GROM.
- ARM coprocessor for SD card support and other stuff. The CPLD is supposed to provide a simple transparent DMA engine, allowing the ARM to inject stuff from SD card to the TI-99/4A's memory expansion, on the fly. I have something like this working on my FPGA system, although that is a much more complex setup than can be done with these parts.
- USB device interface (virtual serial port, running at 12 Mbps or something like that - this piece already works)
- Serial port (3.3V level)
- I2C port
The design is a weird derivative of my current FPGA system. I have connected my previous SD processor board to my FPGA TI-99/4A clone and it already allows me to load ROMs and GROMs from SD cards, my plan is to take that to the point that it would support TI's file system via a DSR. I have already a fair amount of firmware and other bits scattered in my various TI-related projects, so this is one attempt to hopefully gather some of those into a meaningful setup.
That, and the fact that I don't have a 32K memory expansion for my real TI-99/4A, so I wanted to get one and this seems to be what came out...