Aside from trying to move data faster, EEPROM manufacturers offer devices that include features to improve system performance. Examples include Schmitt-trigger inputs to minimize false switching, block-write protection, and password protection.
Password and block-write protection are two important aspects in which designers of networking and communications systems have a vested interest. Various levels of security are available on the chips from EEPROM suppliers. Xicor includes 64-bit password security on its X76F102. The 1024-bit device contains one 112-byte storage array and a 32-byte password storage array that holds two 16-byte user-programmable passwords (one for reads and one for writes). Also included is a retry-counter register, which keeps track of the attempts to access the secured array. If eight attempts are made with invalid passwords, the counter will activate the on-chip erase logic and erase the contents of the main storage array (Fig. 3).
Versions of the X76Fxxx secure serial flash memory are available with capacities up to 8 kbytes. The largest device, the X76F641, can have up to five 64-bit passwords for read, program, and reset control. All of the devices are available in either an 8-lead SOIC package or in a smart-card module.
Secure data products also exist in the form of code-hopping devices. They're employed by the automotive industry for car locks and by other industries for secure access. Microchip, for example, offers a family of KeeLoq encoder and decoder chips that include programmable encryption keys of 64 bits. Seed lengths go from 32 to 60 bits.
When data doesn't have to be secured, but just protected against overwriting, many designers will employ the write-protect capabilities available on most serial EEPROMs. Some devices offer full-chip write protection. Once data is stored, no alterations can take place short of erasing the entire chip. Other, more flexible solutions provide block-write-protect capabilities, which protect one or more sections (blocks) of the memory array once the initial data is written into the block.
Data-Protection Options
Examples that are serial SPI-compatible devices are the M95256 and M95128 from STMicroelectronics (Fig. 4). The 256- and 128-kbit chips offer both hardware and software write protection, a 40-year data-retention guarantee, and an endurance of 100,000 write cycles. Through 2 bits set via software control, the chips can be set to protect none of the memory, the upper quarter, the upper half, or all of the storage array. In the hardware-protection mode, the data bytes in the protected area as well as the contents of the status register are write protected.
A similar capability is available on the 128-kbit X24128 from Xicor. The I2C-compatible memory includes a block-lock capability that resembles the chips from STMicroelectronics. It also includes a 32-word page-write capability, a data-retention guarantee of 100 years, and an endurance of 100,000 cycles.
Along with pure memory functions, a number of companies have leveraged the ability to integrate small amounts of EEPROM storage on a chip with other functions. The result is a wide variety of peripheral support functions: digital potentiometers (DACs with EEPROM-based data-input registers) that remember their last setting, configuration switch replacements, and specialty devices that target markets such as the PC industry.
Check out the mixture of logic and EEPROM in the chip developed by Philips Semiconductors to support Pentium II chip sets. The PCA8550 combines a 4-bit 2:1 multiplexer and a 1-bit latch (Fig. 5). This device replaces mechanical configuration jumpers and switches with a simple-to-configure memory to speed motherboard setup and reduce potential damage from handling.
Housed in a 16-pin SO package, the chip is programmed via its I2C serial bus using a 2-byte sequence. The first serial byte transmitted to the chip contains a 7-bit address and a 1-bit command (Read/Write). This data determines the direction of the data transfer from the chip's nonvolatile register to the host (Read), or from the host to the chip's nonvolatile register (Write). The second byte holds data bits. Depending on the state of the command bit, these bits will supply data to the 5 EEPROM bits that control the multiplexer and latch. Or, they'll capture the state of the EEPROM bits and return that information to the host.
The chip provides a 4-bit 2:1 multiplexer and a 1-bit latch, all controlled by a 5-bit internal, nonvolatile register. With an override pin, the system can force all outputs to logic 0, which aids in trying to perform hardware diagnostics. A write-protect pin lets the host enable/disable I2C writes to the register. The primary function of the 4-bit 2:1 multiplexer is to select either a 4-bit input or data from a nonvolatile register and drive this value onto the output pins. One additional non-multiplexed register output is provided. During I2C writes to the nonvolatile register, that output gets latched to prevent output-value changes.
A Mix Of Capabilities
Digital potentiometers also offer a blend of functionality. A DAC delivers the desired analog output, which is used in turn to control the volume in audio applications. The digital input to the converter comes from a simple 4-, 6-, or 8-bit nonvolatile register that's loaded with the desired value via a serial interface. Several companies list such products, which are available in single, dual, or quad implementations. Catalyst, Dallas Semiconductor, and Xicor are some of the main suppliers.
Microprocessor supervisory circuits with on-chip EEPROM are a novel offering from Catalyst. Aside from precision power-supply monitoring, a watchdog timer, and five programmable reset thresholds, the chips come with 2 to 16 kbits of on-chip EEPROM. They combine two commonly needed functions in microprocessor systems onto a single chip, simplifying system design, further reducing board space, and lowering system cost. Designed to tie into I2C-compatible systems, the CAT24Cxxx series can clock at 400 kHz. Another option is to include a 16-byte page-write buffer to speed data storage.
This broad variety of EEPROM serial memories gives designers a wide latitude for selecting the best solution for their systems. And it only promises to improve in the future, as process technologies and design tools allow the creation of even more versatile products.