New skill for building legacy hardware cicuit mockups

skills/legacy-circuit-mockups/references/28256-eeprom.md

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+# AT28C256 256K (32K x 8) Parallel EEPROM Specification
+
+## 1. Overview
+
+The **AT28C256** is a non-volatile, electrically erasable and programmable read-only memory (EEPROM) manufactured by Atmel (now Microchip). It provides **256 Kbits** of storage organized as **32,768 x 8 bits** and is commonly used in 8-bit microprocessor systems such as those based on the **6502**, **Z80**, and similar CPUs.
+
+The device supports byte-level write operations, fast read access, and software-controlled data protection.
+
+---
+
+## 2. General Characteristics
+
+| Feature        | Description                    |
+| -------------- | ------------------------------ |
+| Memory size    | 256 Kbits (32 KB)              |
+| Organization   | 32,768 x 8 bits                |
+| Data bus       | 8-bit                          |
+| Address bus    | 15-bit (A0-A14)                |
+| Technology     | EEPROM                         |
+| Endurance      | ≥ 100,000 write cycles         |
+| Data retention | ≥ 10 years                     |
+| Access time    | 150-250 ns (variant dependent) |
+| Package types  | DIP-28, PLCC-32, TSOP          |
+
+---
+
+## 3. Pin Configuration (Logical)
+
+### 3.1 Address Pins (A0-A14)
+
+* Select one of 32,768 memory locations
+
+### 3.2 Data Pins (I/O0-I/O7)
+
+* Bidirectional tri-state data bus
+* Outputs valid during read cycles
+
+### 3.3 Control Pins
+
+| Pin | Description                |
+| --- | -------------------------- |
+| CE  | Chip Enable (active low)   |
+| OE  | Output Enable (active low) |
+| WE  | Write Enable (active low)  |
+| VCC | +5 V power supply          |
+| GND | Ground                     |
+
+---
+
+## 4. Memory Organization
+
+* Linear address space from `$0000` to `$7FFF`
+* Each address corresponds to one 8-bit byte
+
+```text
+Address Range: 0000h - 7FFFh
+Data Width:    8 bits
+```
+
+---
+
+## 5. Read Operation
+
+### 5.1 Read Cycle Conditions
+
+| Signal | State |
+| ------ | ----- |
+| CE     | LOW   |
+| OE     | LOW   |
+| WE     | HIGH  |
+
+* Data appears on I/O pins after access time
+* Output remains valid while CE and OE are asserted
+* Outputs are high-impedance when CE or OE is HIGH
+
+---
+
+## 6. Write Operation
+
+### 6.1 Byte Write Cycle
+
+| Signal | State     |
+| ------ | --------- |
+| CE     | LOW       |
+| OE     | HIGH      |
+| WE     | LOW pulse |
+
+* Address and data must be stable during WE low pulse
+* Internal write cycle time ≈ 10 ms (max)
+* Device automatically handles erase-before-write
+
+---
+
+## 7. Software Data Protection (SDP)
+
+The AT28C256 includes optional **Software Data Protection** to prevent accidental writes.
+
+### 7.1 SDP Enable Sequence
+
+```text
+Write $AA to address $5555
+Write $55 to address $2AAA
+Write $A0 to address $5555
+```
+
+### 7.2 SDP Disable Sequence
+
+```text
+Write $AA to address $5555
+Write $55 to address $2AAA
+Write $80 to address $5555
+Write $AA to address $5555
+Write $55 to address $2AAA
+Write $20 to address $5555
+```
+
+---
+
+## 8. Write Cycle Timing Notes
+
+* Writes are internally timed; no external polling required
+* During write cycle, reads return undefined data
+* Device ignores additional write attempts while busy
+
+---
+
+## 9. Data Polling (Optional)
+
+* I/O7 may be monitored during write
+* When I/O7 matches written data, write is complete
+
+---
+
+## 10. Reset and Power Behavior
+
+* No explicit reset pin
+* Writes inhibited during power-up and power-down
+* Outputs default to high-impedance until CE and OE asserted
+
+---
+
+## 11. Typical System Integration (6502 Example)
+
+```text
+Address Range: $8000 - $FFFF
+A15 used as chip select
+OE  R/W?
+WE  inverted R/W?
+```
+
+---
+
+## 12. Absolute Maximum Ratings (Summary)
+
+| Parameter     | Rating                |
+| ------------- | --------------------- |
+| VCC           | -0.6 V to +6.25 V     |
+| Input voltage | -0.6 V to VCC + 0.6 V |
+| Storage temp  | -65 °C to +150 °C     |
+
+---
+
+## 13. Variants and Compatible Devices
+
+| Device           | Notes                        |
+| ---------------- | ---------------------------- |
+| AT28C256         | Original Atmel               |
+| AT28C256F        | Faster access time           |
+| SST28SF256       | Flash-compatible alternative |
+| 28C256 (generic) | Common pin-compatible EEPROM |
+
+---
+
+## 14. Common Use Cases
+
+* ROM replacement in retro systems
+* Firmware storage
+* Microcomputer monitors and BASIC ROMs
+* Prototyping and hobbyist computers
+
+---
+
+## 15. References
+
+* <https://www.utmel.com/components/at28bv256-eeproms-pinout-equivalent-and-datasheet?id=1019>
+* <https://www.futurlec.com/Memory/28C256.shtml>
+* <https://ww1.microchip.com/downloads/en/DeviceDoc/doc0006.pdf>
+* <https://bread80.com/2020/08/10/the-ben-eater-eeprom-programmer-28c256-and-software-data-protection/>
+
+---