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I’m trying to build a cpu out of ttl logic chips. I’ve just about got everything finished, however, I’m still working on the ram. I don’t have any suitable ram chips I can use, however, I do have a couple of these eeprom chips available (link). Can I use this? I probably won’t need to write too much to it, and the cpu will operate at very low frequencies. It is going to be a very basic CPU, infact I’ll probably just be happy if I can get the thing to count. I can’t think of any reason why these won’t work...

http://ww1.microchip.com/downloads/en/DeviceDoc/20002213B.pdf

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  • \$\begingroup\$ it usually won't work because the controller in the eeprom itself typically speaks a serially transported command language, which really means you need a working CPU just to talk to it. But it depends on the EEPROM type – I can imagine some simply actually having address lines. What EEPROM type are we talking about? Link to a datasheet! However, EEPROMs really typically make it significantly more complex to read and write them than RAM ICs, so I doubt you're doing yourself any favor here. \$\endgroup\$ – Marcus Müller Mar 20 at 6:42
  • \$\begingroup\$ Cool, thanks 👍. I’ll add a data sheet now... \$\endgroup\$ – Steve Mar 20 at 6:44
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    \$\begingroup\$ That's an I²C EEPROM – an I²C bus controller able to read arbitrary addresses is quite likely more complex than your whole CPU at this point… \$\endgroup\$ – Marcus Müller Mar 20 at 6:57
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    \$\begingroup\$ Get some RAM or else FeRAM chips. It will simplify things a lot. \$\endgroup\$ – jonk Mar 20 at 7:01
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    \$\begingroup\$ An idea to consider is that at slow clock rates you could use an MCU and its software as your memory. It sounds silly, but a key advantage is that if you let it stop the CPU and it has a serial or USB interface, then you can get in to pre-load, examine, or change things, which is a huge advantage over the tools typically available to a hobbyist trying to bootstrap a TTL computer in the era of TTL. \$\endgroup\$ – Chris Stratton Mar 20 at 8:16
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That's an I²C EEPROM – an I²C bus controller able to read arbitrary addresses is quite likely more complex than your whole CPU at this point.

So, get yourself a simple static RAM IC with separate data and address buses.

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    \$\begingroup\$ In particular, a CMOS static RAM in a 2k x 8 bits configuration used to be available from a large number of manufacturers, and would be perfect for this application. Look for part numbers containing "2016" or "6116". 8k x 8 bits was also quite common -- look for "6264". \$\endgroup\$ – Dave Tweed Mar 20 at 12:38
  • \$\begingroup\$ The 62256 (62C256) is also quite commonly available from even the smaller suppliers. \$\endgroup\$ – Richard the Spacecat Mar 20 at 13:00

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