Interfacing and Programming the Microchip 25LC256-I/ST 64K SPI Serial EEPROM

The Microchip 25LC256-I/ST is a 256-Kbit (32-Kbyte) serial EEPROM featuring the ubiquitous Serial Peripheral Interface (SPI) bus. Its combination of non-volatile memory, high-speed access, and a simple 8-pin package makes it an ideal solution for storing configuration data, calibration constants, or event logging in embedded systems. This article outlines the core concepts for successfully interfacing and programming this versatile memory chip.

Understanding the SPI Interface

SPI is a synchronous, full-duplex serial communication protocol involving a single master device (typically a microcontroller, or MCU) and one or more slave devices (like the 25LC256). The interface relies on four essential signals:

SCK (Serial Clock): Generated by the master to synchronize data bit shifting.

MOSI (Master Out Slave In): The data line from the master to the slave.

MISO (Master In Slave Out): The data line from the slave back to the master.

CS (Chip Select): Activated low by the master to select and enable the target slave device for communication.

The 25LC256 also features HOLD and WP (Write-Protect) pins. The HOLD pin allows the MCU to pause a serial transfer without resetting the communication sequence, which is useful in interrupt-driven systems. The WP pin, when driven low, provides hardware protection for the upper quarter of the memory array (Status Register and addresses 0x7000-0x7FFF), preventing accidental writes.

Core Operational Instructions

Communication with the EEPROM is accomplished by sending a set of instructions followed by an address and/or data. Key instructions include:

WREN (Write Enable Latch): A crucial prerequisite before any write operation. This command (0x06) must be issued to set an internal latch, enabling writes to the memory or status register.

WRDI (Write Disable Latch): Command (0x04) to reset the write enable latch and disable write operations.

READ: Command (0x03) to read data from the memory array. It is followed by a 16-bit address (two bytes), after which the EEPROM will output data on the MISO line.

WRITE: Command (0x02) to write data to the memory array. After the command and 16-bit address, the MCU transmits the data to be written on the MOSI line.

RDSR (Read Status Register): Command (0x05) to read the Status Register. This is vital for checking the WIP (Write-In-Progress) bit, which is set high during a self-timed internal write cycle. Polling this bit is mandatory to ensure a previous write operation is complete before initiating a new one.

Critical Programming Considerations

1. Page Size: The 25LC256 features a 64-byte page buffer. While sequential writes can cross page boundaries, a roll-over will occur within the page, overwriting previous data. Firmware must manage writes to avoid unintentional data corruption across page boundaries.

2. Write Cycle Time: The EEPROM requires a finite time (t_WC), typically 5 ms, to complete the internal programming of a byte or page. During this time, the device will not respond to new commands except for RDSR. Attempting to read or write during this period without checking the WIP bit will fail.

3. Software Control Flow: A robust programming sequence is essential:

For a Write: 1) Assert CS low. 2) Send WREN instruction. 3) Deassert CS high (latching the WREN command). 4) Re-assert CS low. 5) Send WRITE instruction + address + data. 6) Deassert CS high (initiating the internal write cycle). 7) Poll the Status Register (RDSR) until the WIP bit is clear.

For a Read: 1) Assert CS low. 2) Send READ instruction + address. 3) Read data bytes. 4) Deassert CS high.

ICGOOODFIND

The Microchip 25LC256-I/ST is a highly reliable and easy-to-use SPI EEPROM. Successful implementation hinges on a solid understanding of the SPI protocol, strict adherence to the write-enable sequence, and diligent polling of the status register to manage the internal write cycle. By respecting its operational constraints, developers can seamlessly integrate robust non-volatile memory into a wide array of embedded applications.

Keywords: SPI Interface, Non-volatile Memory, Write Cycle, Status Register, Page Buffer