I was motivated by the discussion on EASy68K-compatible, 68000 computer (
viewtopic.php?f=10&t=829) and my recent great experience with a 68302- based single board computer to think seriously about a bare-bone 68302 homebrew. The 68302 is effectively a 68000 (or 68008) integrated with a very powerful I/O processor. Most the features people wanted are inside the 68302 such as timers (3), multiple serial ports (3), SPI master, interrupt controller, discrete I/O, programmable chip selects (which can function as memory remap). The 68302 can be configured like 68008 with byte-wide external memories. So a minimum system can be just a CPU, a RAM and a flash plus an 16MHz oscillator. A quick hand sketched schematic shows there are only about 60 address/data/control connections, 25 power/ground, and about 15 hardware configuration connections. The address/data/control connections can be cut to 30 if the RAM and flash are stacked together, piggy-back fashion, since so many RAM and flash signals have the same pin assignment. All devices are available in DIP and PGA package, so hand wiring 70 or so connections can be done easily in a evening (theoretically speaking

).
I have 128K RAM and flash in DIP package, but I don't have 68302 in PGA. You can buy them on eBay for under $20, but I thought I'll try with a quad flat pack 68302 that I removed from a 68302 SBC. (I removed all components so I can reverse engineer the pc board connections). The lead spacing of the QFP is 25 mil, so I have to work under a microscope and it will take more than an evening to wire 60 or so connections to the QFP.
I want to have a solid digital ground, so all components will be mounted on a single-sided copper board, roughly 2"x5". Most of the copper foil is digital ground except areas carved out for other signals such as pads for the piggy-back RAM and flash. The piggy-back RAM/flash is soldered to the board because I need solid mechanical support to insert and remove the flash many times (picture 1).
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File comment: Piggy back RAM and Flash socket
DSC_20761228_F.jpg [ 240.55 KiB | Viewed 11432 times ]
The QFP 68302 was glue to the board, dead-bug fashion. Ground leads are bend down toward the copper foil for short ground connections and Vcc leads bend toward the center where a 5V ring will be constructed. A third of way into wiring, after ground/power were connected and all hardware configuration finished, I connected the 16MHz oscillator and power up the CPU with oscillator. I can see chip select 0 puts out 8 successive pulses to fetch reset vector and system stack and repeat every 1 millisecond. That means the CPU is probably OK, even though I'd treated it rather badly when removing with a heat gun. Soldering 60 or so connection to the QFP 68302 was challenging and it did take more than an evening. I know I can do it 10 years ago and I'm glad I still can do it. Nevertheless I'll stick with the PGA package on the next iteration!
Photo 2 shows the finished board. It can fetch reset vector and execute an endless loop of NOP so flash is working. I'll need to test the RAM next and then bring up a simple bootloader and then the EASy68K compatible monitor!
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File comment: Completed Tiny302
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