AT24C128C-XHM-T: A Deep Dive into the 128-Kb I2C Serial EEPROM
The AT24C128C-XHM-T from Microchip Technology is a cornerstone component in modern electronics, providing reliable non-volatile memory storage for a vast array of applications. As a 128-kilobit (16-kilobyte) Serial Electrically Erasable Programmable Read-Only Memory (EEPROM) that communicates via the I2C protocol, it offers a compelling blend of density, simplicity, and performance. This article explores its key features and critical design considerations for successful implementation.
Salient Features of the AT24C128C-XHM-T
The device's popularity is rooted in a robust feature set designed for system flexibility and resilience.
High-Density Storage: Organized as 16,384 words of 8 bits each, the 128-Kbit capacity is sufficient for storing substantial data sets, such as device configuration parameters, user settings, calibration data, and event logs.
I2C-Compatible Interface: The two-wire serial interface (Serial Clock - SCL and Serial Data - SDA) drastically reduces system pin count and simplifies board layout compared to parallel memories. It supports a maximum clock frequency of 1MHz (1.8V to 5.5V), enabling high-speed data transfers.
Wide Voltage Operation: With an operating voltage range from 1.7V to 5.5V, it is compatible with various logic levels, from modern low-power microcontrollers to legacy 5V systems, without needing a level translator.
Advanced Write Protection: The device includes both software and hardware write protection mechanisms. The WP (Write Protect) pin, when held high, protects the entire memory array from inadvertent writes. Software write control allows for protecting quarter, half, or the full array.
Enhanced Reliability: Rated for 1,000,000 write cycles and featuring a 100-year data retention capability, this EEPROM is built for enduring performance in critical applications.
Critical Application Design Considerations
While simple to integrate, careful attention to certain design aspects is paramount for robust operation.
1. I2C Pull-Up Resistor Calculation: The I2C bus requires external pull-up resistors on both SCL and SDA lines. Their value is a critical trade-off. Values that are too low cause excessive current draw, while values that are too high lead to slow rise times and signal integrity issues. The resistor value (Rp) must be calculated based on the bus capacitance (Cb), supply voltage (Vcc), and the desired rise time (tr) using the formula: `tr = 0.8473 Rp Cb`. Typical values range from 1kΩ for fast, low-capacitance buses to 10kΩ for standard-mode operations.
2. Page Write Limitations: The memory is organized into 64-byte pages. While the host can write up to 128 bytes in a single write cycle, a write operation cannot cross a physical page boundary. If a sequential write attempts to cross from page `n` (ending at address 63) to page `n+1` (starting at address 64), the address pointer will wrap around to the start of the same page (`n`, address 0), overwriting previously written data. The firmware must manage data alignment to prevent this.
3. Acknowledge Polling: After issuing a write command, the device internally initiates the write cycle (tWR) and will not acknowledge further commands until it is complete. This period typically lasts up to 5ms. The most efficient way to proceed is to use acknowledge polling: the master continues to send a START condition followed by the device slave address (for a write operation) until the EEPROM acknowledges, signaling the write cycle is finished and it is ready for new commands.
4. Hardware Addressing and Device ID: The address pins (A1, A2) allow up to four devices to be connected on the same bus. The least significant four bits of the 7-bit slave address are fixed as '1010', followed by the state of pins A2 and A1, and finally the R/W bit. Modern versions also incorporate an 8-byte Unique Device ID (UID) factory-programmed into the memory, which is invaluable for secure identification and anti-cloning strategies.
5. Power Supply Decoupling: Like all high-speed integrated circuits, proper power supply decoupling is essential. A 100nF ceramic capacitor should be placed as close as possible to the VCC and GND pins of the EEPROM to filter high-frequency noise and ensure stable operation during write cycles.
ICGOOODFIND
The AT24C128C-XHM-T stands out as an industry-standard solution for auxiliary memory needs. Its combination of a simple I2C interface, substantial density, and robust data integrity features makes it an excellent choice for designers. By meticulously addressing pull-up resistor selection, page write boundaries, and communication protocols, engineers can fully leverage its capabilities to create reliable and efficient products across consumer, industrial, and automotive domains.
Keywords:
I2C EEPROM
Non-volatile Memory
Page Write
Acknowledge Polling
Hardware Addressing
