EEPROM Interfencing and Programming with the Microchip 93LC46B/P
Non-volatile memory is a cornerstone of embedded systems design, allowing critical data to be retained even when system power is removed. Among the various options available, serial EEPROMs offer an excellent combination of density, cost-effectiveness, and a simple interface. The Microchip 93LC46B/P is a classic and widely adopted 1K-bit serial Electrically Erasable Programmable Read-Only Memory (EEPROM) that serves as a perfect introduction to this technology. This article explores its interface and programming fundamentals.
The 93LC46B is organized as 128 x 8-bit or 64 x 16-bit, providing flexibility for byte- or word-oriented systems. It communicates via a simple 3-wire serial interface, comprising a Chip Select (CS), a Clock (SK), and a bidirectional Data Input/Output (DI/DO) line. This minimal pin count makes it ideal for space-constrained designs and helps free up valuable microcontroller I/O pins.
Communication with the 93LC46B is a precise, synchronous process governed by a specific instruction set. All operations, whether reading or writing, begin with a Start Bit (a high transition on DI with CS high and SK high), followed by an 8-bit or 16-bit instruction opcode. This opcode defines the operation (Read, Write, Erase, Write Enable, Write Disable, Erase All) and, for certain instructions, the memory address. Key operations include:
Read (READ): The microcontroller sends the READ opcode and an address. The EEPROM then outputs the data from the specified location on the DO line.
Write (WRITE): This requires a two-step process. First, the Write Enable (EWEN) instruction must be issued to disable the internal write protection. Then, the WRITE opcode, address, and the data word are sent. The chip then enters a self-timed programming cycle (typically 3-4 ms) during which it will not respond to commands.
Erase (ERASE): This instruction clears a single memory location (writes all bits to a '1' state), preparing it for a new value.
Erase All (ERAL): A powerful command that erases the entire memory array in a single operation, useful for mass initialization but requiring careful use.
A critical aspect of programming the 93LC46B is polling for completion. After initiating a Write or Erase command, the microcontroller must wait for the self-timed cycle to finish before sending a new command. This is efficiently done by polling the device's ready status. After the command is sent, the microcontroller can toggle the clock (SK) with CS high. The DO pin will remain in a high-impedance state until the internal cycle is complete, at which point it will output a '0' (low) ready signal, indicating the device is prepared for the next instruction. Failing to observe this wait period is a common source of data corruption.
Robustness and data integrity are further enhanced by built-in features. The commit to memory is only finalized by a specific high-to-low transition of the CS pin at the end of the instruction sequence. An inadvertent power loss or glitch during transmission that prevents this proper termination will typically abort the operation. Furthermore, always concluding a write session with a Write Disable (EWDS) instruction is a recommended best practice to re-enable write protection and prevent accidental data modification.
Keywords: EEPROM, 93LC46B, Serial Interface, Non-volatile Memory, Write Cycle
