When exposed to an electrical charge, an Electrically Erasable Programmable Read-Only Memory, or EEPROM, can be erased. However, it does maintain the data even when the power is turned off to the device. EEPROM is different from flash memory in that, with EEPROM the data is either written or erased one byte at a time so the process is slow. With flash memory, whole blocks of data are written and erased simultaneously, speeding up the process.
The programming or erasing of EEPROMs can be accomplished in-circuit using special programming signals. Advantages of EEPROMs include that they allow multi-byte page operations, offer low pin count, small packages, low-voltage operation and low power consumption. Their lives have been dramatically extended from the handful of times it could be reprogrammed to the current up to a million write operations in modern EEPROMs. In order to save board space, serial EEPROMs can be integrated into logic devices so that they can be present on a board for high reliability yet save space and total system cost.
The devices are not without challenges, however. Electronic systems are not getting larger; they continue to be found on smaller and smaller boards. The programming and support circuitry required for EEPROMs add to total system cost, and since the fewer the number of board devices, the lower the total power consumption and higher the reliability. Also, EEPROMs can become obsolete over time requiring time-consuming redesigns due to process technology advances.
There are two main categories of EEPROMS, serial and parallel.
Serial EEPROMs communicate through a serial interface and operate in op-code, address and data phases. These three interfaces use one to four control signals to operate so the EEPROM requires up to an 8-pin package. These EEPROMs support several read/write operations, as well as program, sector erase and chip erase commands.
One example is the Atmel AT24C32E-SSHM-B Serial EEPROM, an I2C compatible surface-mount device that features low-voltage operation from 1.7 V to 3.6 V, a standard mode of 100 kHz, fast mode of 400 kHz, also at 1.7 V to 3.6 V and a 1 MHz Fast-Mode Plus (FM+) at 2.5 V to 3.6 V. It also features Schmitt trigger-filtered inputs for noise suppression, bidirectional data transfer protocol, write protect pin for full array hardware data protection and an ultra-low active current (1 mA maximum) and standby current (0.8 μA maximum). It features high reliability, an endurance of 1,000,000 write cycles and data retention of 100 years. Applications include many industrial and commercial applications where low-power and low-voltage operation is necessary.
Figure 1: The Atmel AT24C32E-SSHM-B Serial EEPROM Block Diagram. (Source: Atmel)
Another example of a serial EEPROM is the Microchip 24LC04BT-I/SN16KVAO EEPROM with operation of 1.7 V or 2.5 V. Based on low-power CMOS technology, it features a read current of 1 mA (typical) and standby current of 1 μA (typical). It also has Schmitt Trigger inputs for noise suppression as well as output slope control to eliminate ground bounce and 100 kHz and 400 kHz clock compatibility. ESD protection of the device is >4,000 V. It has more than 1 million erase/write cycles and data retention of more than 200 years. Applications include industrial and automotive.
In comparison, parallel EEPROMs are non-volatile memories that communicate through a parallel interface, have an 8-bit data bus and an address bus sufficiently wide to cover the complete memory. Most parallel EEPROMs feature chip-select and write-protect pins. Microcontrollers can have integrated parallel EEPROMs. While they are faster than serial EEPROMs, they are larger based on a higher pin count of 28 or more pins, often meaning they are used less often as end-products shrink to smaller form factors.
An example is the 5962-8751408XA parallel 64 K-bit 8K x 8 5 V 28-Pin EEPROM by e2v that has a typical operating supply voltage of 5 V and operating current of 80 mA. It features data retention of 10 years, through-hole mounting and a 28-pin count.