EEPROM Memory: Interfacing and Programming the Microchip 93LC46A-I/P Serial Chip
In the world of embedded systems and electronics, the need for non-volatile memory—storage that retains data even when power is removed—is fundamental. While Flash memory is common for larger capacities, Serial EEPROMs like the Microchip 93LC46A-I/P offer a simple, efficient, and cost-effective solution for storing smaller amounts of critical data, such as configuration settings, calibration constants, or user preferences. This article explores the interfacing and programming of this versatile chip.
The 93LC46A is a 1K-bit (128 x 8 or 64 x 16) CMOS serial Electrically Erasable PROM. Its key advantage lies in its simple 3-wire serial interface (plus a chip select line), which requires very few I/O pins from a host microcontroller (MCU), making it ideal for space-constrained designs. It supports both a Microwire synchronous protocol and a subset of the SPI protocol, offering flexibility for integration.
Key Features of the 93LC46A-I/P:
Organization: 128 x 8-bit or 64 x 16-bit (selectable via an ORG pin).
Voltage Range: 2.5V to 5.5V, suitable for both 3.3V and 5V systems.
Interface: Microwire/SPI-compatible serial interface (CS, SK, DI, DO).
Write Protection: Built-in write-protect control via software commands and a dedicated pin.
Sequential Read: Allows for efficient continuous read operations.
Interfacing with a Microcontroller
The physical connection between an MCU and the 93LC46A is straightforward. The four essential pins are:
1. CS (Chip Select): Driven by the MCU, this pin must be held high for any operation to occur. It frames the command and data packets.
2. SK (Serial Clock): Generated by the MCU, this clock signal synchronizes the data shifting in and out of the chip.
3. DI (Data In): The MCU uses this line to send opcodes and data to the EEPROM.
4. DO (Data Out): The EEPROM uses this line to send data back to the MCU during read operations.
A typical connection diagram would show these four lines connected to any general-purpose I/O (GPIO) pins on the MCU. While it's possible to use hardware SPI peripherals, the protocol's slight differences often make bit-banging (software-controlled GPIO toggling) a simpler and more common approach.
Programming and Command Set
All communication is initiated by the MCU sending a start bit (a high on DI with CS high and a rising edge on SK), followed by an opcode that defines the operation. The 93LC46A understands a small set of crucial commands:
READ (OPCODE 10): The MCU sends the address, and the EEPROM responds by outputting the data word on the DO pin.
WRITE (OPCODE 01): The MCU sends an address followed by the data to be written. This is a critical process.
ERASE (OPCODE 11): Erases a specific memory location (sets all bits to '1'), which is a prerequisite for a write operation in EEPROM technology.
EWEN (Erase/Write Enable - OPCODE 00, 11): A specific sequence that must be sent before any modify (erase/write) operation to prevent accidental data corruption.
ERAL (Erase All - OPCODE 00, 10): Erases the entire memory array.
WRAL (Write All - OPCODE 00, 01): Writes the same data to all memory locations.
The Critical Write Cycle
A key aspect of programming any EEPROM is understanding the write cycle time. After receiving the last bit of a WRITE or ERASE command, the 93LC46A enters an internal self-timed programming cycle (`t_{WP}`), typically lasting up to 5ms. During this time, the chip will not respond to commands. The MCU must poll the DO pin after issuing the command; the pin is in a high-impedance state until the write cycle completes, at which point it goes active, signaling the MCU that the chip is ready for a new command. Failing to wait for this completion is a common source of data write errors.
Conclusion and Application
The Microchip 93LC46A-I/P exemplifies the elegance of simple, dedicated-purpose ICs. Its minimal pin count, low power consumption, and robust command set make it a perennial favorite for designers needing reliable, small-scale non-volatile storage. Successfully integrating it into a project requires careful attention to the timing diagrams and, most importantly, proper management of the write cycle.
Keywords: Serial EEPROM, Microwire Interface, Non-Volatile Memory, Write Cycle Time, Bit-Banging
