I can't find the list message for this, but I seem to recall a long time ago someone inquired about figuring out how to make a ROM programmable when there is no write signal, going on to suggest using read accesses to perform the write activity. Maybe I am dreaming the conversation, but it came from somewhere, and I decided a long time ago to create such a device. My first attempt in 2018 did not work, and I'd built it using SMT parts so it was hard to debug. I figured I needed an external clock to sync things, so I finally designed a non SMT version with an external clock. I won't rule out the need for a clock signal, but it turns out the issue was my (incorrect) assumption about FLASH ICs and their /OE line. If someone else decides to use the SST 39SFXXX FLASH ROMs, note that the /OE line must be high for a write to succeed. Anyway, probably no one asked for this, but I wanted to see if it would work, and it does: https://github.com/go4retro/WriteROM The device has a CPLD and a 512kB FLASH ROM. The unit starts in normal ROM mode, but you can switch on programming by reading from specific addresses. In programming mode, address lines 11:8 form the command, and 7:0 are the data. Commands include: * set low, mid, high bytes of stored address * read at stored address * write to stored address The unit has 2 size jumpers, allowing it to function as a 8/16/32/64kB ROM. Any address line not connected to the bus can be set by the commands above. Currently, the prototype is a 28 pin footprint (23128/256), but I will probably create a 2364 footprint variant or make one PCB that can be configured either way. If anyone sees improvements or other ideas, let me know. Jim -- Jim Brain brain_at_jbrain.com www.jbrain.comReceived on 2025-09-05 03:00:01
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