AT28C256 150ns Parallel EEPROM: Detailed Pinout, SDP Unlocking, and Retrocomputing Design Guide

Executive Summary: What is the AT28C256?

The AT28C256 is a 256-Kbit (32K x 8) paged parallel EEPROM designed for high-reliability nonvolatile storage in industrial, military, and legacy computing systems. It offers a robust alternative to older EPROM technology by allowing in-system electrical reprogramming without the need for UV erasure.

• Market Position: High-reliability legacy standard; the go-to choice for 8-bit bus architectures.

• Top Features: 150 ns fast access time, 5V single power supply, and internal Software Data Protection (SDP).

• Primary Audience: Ideal for retrocomputing enthusiasts (Z80/6502), industrial maintenance engineers, and aerospace hardware designers.

• Supply Status: Active (Manufactured by Microchip Technology).

1. Technical Specifications & Performance Analysis

1.1 Core Architecture (Parallel EEPROM)

The AT28C256 utilizes a paged architecture, specifically organized as 32,768 words by 8 bits. Unlike serial EEPROMs that use I2C or SPI, this parallel interface allows the CPU to access memory addresses directly on the data bus, making it significantly faster for executing code directly from the chip (Execute-In-Place).

1.2 Key Electrical Characteristics

• Supply Voltage: Operates at a standard 5V ±10%, making it natively compatible with TTL logic levels.

• Access Speed: Features a 150 ns read access time, which is sufficient for most vintage CPUs running up to 6-8 MHz without wait states.

• Power Consumption: Active current is typically around 30 mA to 45 mA, with a CMOS standby mode that drops significantly to reduce BOM power budgets.

• Endurance: Rated for 100,000 write cycles with a 10-year data retention span, exceeding the requirements for firmware storage.

1.3 Interfaces and Connectivity

The device features a standard parallel interface consisting of 15 address lines (A0-A14) and 8 bi-directional data lines (I/O0-I/O7). Control is managed via three active-low pins: Chip Enable (CE#), Output Enable (OE#), and Write Enable (WE#).

2. Pinout, Package, and Configuration

2.1 Pin Configuration Guide

• A0 – A14 (Address Inputs): Selects the specific 8-bit byte within the 32K memory space.

• I/O0 – I/O7 (Data Inputs/Outputs): The 8-bit bi-directional data bus.

• WE# (Write Enable): Must be pulsed low to initiate a write cycle.

• CE# (Chip Enable): Must be low for the device to be active.

• OE# (Output Enable): Gates the data onto the bus during a read cycle.

2.2 Naming Convention & Ordering Codes

Microchip uses suffixes to denote speed and package. For example, AT28C256-15PU indicates: 

- 15: 150 ns access time. 

- P: PDIP (Plastic Dual In-line Package). 

- U: Lead-free/RoHS compliant.

2.3 Available Packages

3. Design & Integration Guide (For Engineers & Makers)

Pro Tip: Always verify pin compatibility before migrating from older series. The AT28C256 is often mistaken for a 27C256 EPROM, but their pinouts differ significantly at pins 1 and 27.

3.1 Hardware Implementation

• Bypass Capacitors: Place a 0.1µF ceramic capacitor as close to the VCC pin as possible to suppress high-frequency noise during switching.

• PCB Layout: Keep address and data bus traces relatively equal in length to prevent timing skew, though at 150ns, this is less critical than in modern DDR memory.

• Pull-up Resistors: It is good practice to place a 10kΩ pull-up resistor on the WE# line to prevent accidental writes during microcontroller power-up sequences.

3.2 Common Design Challenges

• The SDP Lock Issue: Many AT28C256 chips arrive with Software Data Protection enabled.

• Issue: Standard "bit-banging" code on an Arduino is often too slow to perform the 3-byte unlock sequence (requires timing < 150µs).

• Fix: Use direct port manipulation (e.g., PORTD on AVR) or a dedicated programmer like the TL866II Plus to disable SDP.

• 27C256 Incompatibility:

• Issue: Pin 1 is A14 on the AT28C256, but VPP on the 27C256. Pin 27 is WE# on the AT28C256, but A14 on the 27C256.

• Fix: If replacing a 27C256, you must use an adapter or modify PCB traces to avoid grounding your highest address bit.

4. Typical Applications & Use Cases

📺 Recommended Video Tutorial:

4.1 Real-World Example: 8-Bit "Homebrew" Computer

In a Z80 or 6502 computer build, the AT28C256 acts as the "ROM." It stores the OS monitor or BASIC interpreter. Because it is an EEPROM, the designer can update the BIOS code directly from the computer itself using a small assembly routine, rather than pulling the chip and using a UV eraser.

5. Alternatives and Cross-Reference Guide

• Direct Replacements: ON Semiconductor CAT28C256 is a near-identical functional equivalent.

• Legacy Equivalents: Xicor X28C256 was the original standard but is now largely obsolete/surplus only.

• Flash Alternative: The SST39SF010A is a faster NOR Flash alternative, though it requires a different write algorithm and has a different pinout (32-pin).

• EPROM Replacement: If you need a pin-compatible replacement for a 27C256 EPROM, consider the Winbond W27C257, which is electrically erasable but maintains the 27-series pinout.

6. Frequently Asked Questions (FAQ)

• Q: What is the difference between AT28C256 and 27C256?

• A: The AT28C256 is an EEPROM (Electrically Erasable), while the 27C256 is an EPROM (usually UV Erasable). They have different pinouts for pins 1 and 27.

• Q: Can AT28C256 be used in Automotive environments?

• A: Yes, provided you select the "Industrial" or "Automotive" grade versions (suffix 'I' or 'E') which support wider temperature ranges (-40°C to +125°C).

• Q: How do I program the AT28C256 with an Arduino?

• A: You can use an Arduino Mega (due to the high number of I/O pins needed), but you must use direct port writing to meet the timing requirements if SDP is enabled.

• Q: Is the AT28C256 suitable for battery-operated devices?

• A: It is excellent for data retention without power, but during active read/write, it consumes significant current compared to modern low-power Serial Flash.

7. Resources

• Datasheet: Refer to the Microchip website for the latest AT28C256 PDF.

• Development Tools: TL866II Plus Programmer, Ben Eater's EEPROM Programmer project.

• Libraries: Arduino "EEPROM" libraries specifically modified for Parallel interfaces.