Things haven't gone well today at all. No progress has been made on the computer, despite devoting quite a chunk of time to working on it. Here's what's going on:
For starters, the 28C64 doesn't seem to be working at all. It's brand new, with a recent date code, but still refuses to be programmed. As I mentioned, my little eeprom programmer worked great on a 8kb sram chip with the same pinout, but refuses to write. Dr. Jefyll of the 6502.org forum pointed out that write is disabled if any of the enable inputs are tied (that's my understanding, anyway), but all are controlled by the 74LS00 on my circuit, and seem to be correct based on the rams' working fine. Assuming at this point the chip is dead..
Not willing to wait another long while to get new chips, I decided to make a simple nvram circuit. It's based on a 32k sram chip disabled to 8kb, with a 3v coin cell on the back, tied to the VCC pin through a diode, which is then tied to the programmer power with another diode. A 10k resistor also holds /CE high, to put it in standby mode. The datasheet specifies a current draw of 250 microwatts at standby, and data retention down to 2v. I assembled everything, programmed some $AA and $FF, read them back, then restarted the computer. All data gone. I also noticed the battery is running down much quicker than my 5,000+ hour estimate too (not while plugged in, the programmer takes over the power just fine, raising it to about 4.5v). Bother. I don't at all understand why this isn't working, all the voltages are within spec, and the chip works fine plugged in. Still looking in to it. Any thoughts?
Must trudge on..
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