47 lines
2.2 KiB
Markdown
47 lines
2.2 KiB
Markdown
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# 27C256 EPROM — pinout reference for `rom-32k.c`
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The 27C256 (and the older 2764/27256) is the canonical 32 KB EPROM
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used on virtually every 8-bit retro-computer of the late 1970s–80s.
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Datasheet is mirrored extensively (Atmel, ST, AMD, Microchip — all
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permissively redistributable spec sheets, dating from the 1980s).
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## Package: 28-pin DIP
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| Pin (real) | Name | Direction | Notes |
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|------------|------|-----------|-------|
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| 1 | VPP | input | Programming voltage (12V/12.75V). **Tied to VCC for read mode.** |
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| 2–10 | A12, A7, A6, A5, A4, A3, A2, A1, A0 | input | Address — physical pin order is non-trivial |
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| 11–13, 15–19 | D0..D7 | I/O | Data bus (output during read; input during programming) |
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| 14 | GND | power | |
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| 20 | CE̅ | input | Active-low chip enable |
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| 21 | A10 | input | |
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| 22 | OE̅ | input | Active-low output enable |
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| 23–27 | A11, A9, A8, A13, A14 | input | Address remainder |
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| 28 | VCC | power | +5 V |
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## Read-mode logic (from any 27C256 datasheet, "Operating Modes" section)
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| CE̅ | OE̅ | VPP | Mode | Output (D0..D7) |
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|-----|-----|------|------------------|------------------|
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| H | x | VCC | **Standby** | High-Z (tristate) |
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| L | H | VCC | **Output disable** | High-Z |
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| L | L | VCC | **Read** | data byte from `mem[A14..A0]` |
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Velxio is digital-only with no tristate state, so "High-Z" is modelled
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by switching D pins to `VX_INPUT` (the pin retains its last value but
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the chip is not driving). Tests assume that when CE̅ or OE̅ is
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deasserted, the chip will not contend with another driver — verified
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by `rom-32k.test.js`'s "tristate" assertions.
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## Mapping for our `rom-32k.c`
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We collapse the 28-pin package to a clean 27-named-pin model: the
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test's pin contract names every signal and ignores the physical pin
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ordering. Programming-mode pins (PGM̅, VPP behaviour) are out of
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scope — this is a read-only model.
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The first 16 bytes of the embedded ROM image are the test fixture
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`{0x12, 0x34, 0x56, 0x78, 0x9A, 0xBC, 0xDE, 0xF0, 0x11, 0x22, 0x33,
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0x44, 0x55, 0x66, 0x77, 0x88}`; everything else is `0xFF` to mirror
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real-EPROM erased state.
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