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