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62 lines
2.5 KiB
Plaintext
62 lines
2.5 KiB
Plaintext
# Writing to a AT28 from Collapse OS
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# Gathering parts
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* A RC2014 Classic
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* An extra AT28C64B
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* 1x 40106 inverter gates
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* Proto board, RC2014 header pins, wires, IC sockets, etc.
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# Building the EEPROM holder
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The AT28 is SRAM compatible so you could use a RAM module for
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it. However, there is only one RAM module with the Classic
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version of the RC2014 and we need it to run Collapse OS.
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You could probably use the 64K RAM module for this purpose, but
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I don't have one and I haven't tried it. For this recipe, I
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built my own module which is the same as the regular ROM module
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but with WR wired and geared for address range 0x2000-0x3fff.
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If you're tempted by the idea of hacking your existing RC2014
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ROM module by wiring WR and write directly to the range
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0x0000-0x1fff while running it, be aware that it's not that
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easy. I was also tempted by this idea, tried it, but on bootup,
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it seems that some random WR triggers happen and it corrupts the
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EEPROM contents. Theoretically, we could go around that by
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putting the AT28 in write protection mode, but I preferred
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building my own module.
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I don't think you need a schematic. It's really simple.
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# Writing contents to the AT28
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If you wait 10ms between each byte you write, you can write dir-
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ectly to the AT28 with regular memory access words. If you don't
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wait, the AT28 writing program will fail. Because it's not very
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pratical to insert waiting time between each byte writes, you
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need another solution.
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To that end, Collapse OS has a "memory write override" mech-
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anism. Whenever C! or ! is about to set a byte somewhere in
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memory, it checks whether such an override is active. If it is,
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it calls it. That override is set with the "~C!" word.
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B400 contains an override routine compatible with ~C! called
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~AT28. When you're about to write to your AT28, activate that
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override with "' ~AT28 ~C!". That overwrite will write the byte,
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then poll the AT28 until it indicates that it is finished
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writing. This ensures that Collapse OS doesn't try writing
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another byte before the AT28 is ready.
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When you're done writing to the AT28, unset override with
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"0 ~C!". The override routine has a non-negligible speed impact
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on all memory writes.
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When polling, ~AT28 also verifies that the final byte in memory
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is the same as the byte written. If it's not, it will place a
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non-zero value in the ~C!ERR 1b variable. Therefore, if you want
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to see, after a big write operation to your AT28, whether any
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write failed, do "~C!ERR C@ .". Re-initialize to zero before
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your next write operation.
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